[SPARK-13583][CORE][STREAMING] Remove unused imports and add checkstyle rule

## What changes were proposed in this pull request?

After SPARK-6990, `dev/lint-java` keeps Java code healthy and helps PR review by saving much time.
This issue aims remove unused imports from Java/Scala code and add `UnusedImports` checkstyle rule to help developers.

## How was this patch tested?
```
./dev/lint-java
./build/sbt compile
```

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #11438 from dongjoon-hyun/SPARK-13583.

10 files changed, 0 insertions, 15 deletions

diff --git a/common/network-common/src/main/java/org/apache/spark/network/protocol/OneWayMessage.java b/common/network-common/src/main/java/org/apache/spark/network/protocol/OneWayMessage.java
index efe0470f35..f7ffb1bd49 100644
--- a/common/network-common/src/main/java/org/apache/spark/network/protocol/OneWayMessage.java
+++ b/common/network-common/src/main/java/org/apache/spark/network/protocol/OneWayMessage.java
@@ -19,7 +19,6 @@ package org.apache.spark.network.protocol;
 
 import com.google.common.base.Objects;
 import io.netty.buffer.ByteBuf;
-import io.netty.buffer.Unpooled;
 
 import org.apache.spark.network.buffer.ManagedBuffer;
 import org.apache.spark.network.buffer.NettyManagedBuffer;
diff --git a/common/network-common/src/main/java/org/apache/spark/network/protocol/RpcRequest.java b/common/network-common/src/main/java/org/apache/spark/network/protocol/RpcRequest.java
index 96213794a8..2b30920f05 100644
--- a/common/network-common/src/main/java/org/apache/spark/network/protocol/RpcRequest.java
+++ b/common/network-common/src/main/java/org/apache/spark/network/protocol/RpcRequest.java
@@ -19,7 +19,6 @@ package org.apache.spark.network.protocol;
 
 import com.google.common.base.Objects;
 import io.netty.buffer.ByteBuf;
-import io.netty.buffer.Unpooled;
 
 import org.apache.spark.network.buffer.ManagedBuffer;
 import org.apache.spark.network.buffer.NettyManagedBuffer;
diff --git a/common/network-common/src/main/java/org/apache/spark/network/protocol/RpcResponse.java b/common/network-common/src/main/java/org/apache/spark/network/protocol/RpcResponse.java
index bae866e14a..d73014ecd8 100644
--- a/common/network-common/src/main/java/org/apache/spark/network/protocol/RpcResponse.java
+++ b/common/network-common/src/main/java/org/apache/spark/network/protocol/RpcResponse.java
@@ -19,7 +19,6 @@ package org.apache.spark.network.protocol;
 
 import com.google.common.base.Objects;
 import io.netty.buffer.ByteBuf;
-import io.netty.buffer.Unpooled;
 
 import org.apache.spark.network.buffer.ManagedBuffer;
 import org.apache.spark.network.buffer.NettyManagedBuffer;
diff --git a/common/network-common/src/main/java/org/apache/spark/network/protocol/StreamFailure.java b/common/network-common/src/main/java/org/apache/spark/network/protocol/StreamFailure.java
index 26747ee55b..258ef81c67 100644
--- a/common/network-common/src/main/java/org/apache/spark/network/protocol/StreamFailure.java
+++ b/common/network-common/src/main/java/org/apache/spark/network/protocol/StreamFailure.java
@@ -20,9 +20,6 @@ package org.apache.spark.network.protocol;
 import com.google.common.base.Objects;
 import io.netty.buffer.ByteBuf;
 
-import org.apache.spark.network.buffer.ManagedBuffer;
-import org.apache.spark.network.buffer.NettyManagedBuffer;
-
 /**
  * Message indicating an error when transferring a stream.
  */
diff --git a/common/network-common/src/main/java/org/apache/spark/network/protocol/StreamRequest.java b/common/network-common/src/main/java/org/apache/spark/network/protocol/StreamRequest.java
index 35af5a84ba..dc183c043e 100644
--- a/common/network-common/src/main/java/org/apache/spark/network/protocol/StreamRequest.java
+++ b/common/network-common/src/main/java/org/apache/spark/network/protocol/StreamRequest.java
@@ -20,9 +20,6 @@ package org.apache.spark.network.protocol;
 import com.google.common.base.Objects;
 import io.netty.buffer.ByteBuf;
 
-import org.apache.spark.network.buffer.ManagedBuffer;
-import org.apache.spark.network.buffer.NettyManagedBuffer;
-
 /**
  * Request to stream data from the remote end.
  * <p>
diff --git a/common/network-common/src/main/java/org/apache/spark/network/protocol/StreamResponse.java b/common/network-common/src/main/java/org/apache/spark/network/protocol/StreamResponse.java
index 51b899930f..87e212f3e1 100644
--- a/common/network-common/src/main/java/org/apache/spark/network/protocol/StreamResponse.java
+++ b/common/network-common/src/main/java/org/apache/spark/network/protocol/StreamResponse.java
@@ -21,7 +21,6 @@ import com.google.common.base.Objects;
 import io.netty.buffer.ByteBuf;
 
 import org.apache.spark.network.buffer.ManagedBuffer;
-import org.apache.spark.network.buffer.NettyManagedBuffer;
 
 /**
  * Response to {@link StreamRequest} when the stream has been successfully opened.
diff --git a/common/network-common/src/main/java/org/apache/spark/network/sasl/SaslEncryption.java b/common/network-common/src/main/java/org/apache/spark/network/sasl/SaslEncryption.java
index 127335e4d3..3d71ebaa7e 100644
--- a/common/network-common/src/main/java/org/apache/spark/network/sasl/SaslEncryption.java
+++ b/common/network-common/src/main/java/org/apache/spark/network/sasl/SaslEncryption.java
@@ -33,7 +33,6 @@ import io.netty.channel.ChannelPromise;
 import io.netty.channel.FileRegion;
 import io.netty.handler.codec.MessageToMessageDecoder;
 import io.netty.util.AbstractReferenceCounted;
-import io.netty.util.ReferenceCountUtil;
 
 import org.apache.spark.network.util.ByteArrayWritableChannel;
 import org.apache.spark.network.util.NettyUtils;
diff --git a/common/network-common/src/main/java/org/apache/spark/network/server/TransportRequestHandler.java b/common/network-common/src/main/java/org/apache/spark/network/server/TransportRequestHandler.java
index 296ced3db0..bebe88ec5d 100644
--- a/common/network-common/src/main/java/org/apache/spark/network/server/TransportRequestHandler.java
+++ b/common/network-common/src/main/java/org/apache/spark/network/server/TransportRequestHandler.java
@@ -19,7 +19,6 @@ package org.apache.spark.network.server;
 
 import java.nio.ByteBuffer;
 
-import com.google.common.base.Preconditions;
 import com.google.common.base.Throwables;
 import io.netty.channel.Channel;
 import io.netty.channel.ChannelFuture;
diff --git a/common/network-common/src/main/java/org/apache/spark/network/util/NettyUtils.java b/common/network-common/src/main/java/org/apache/spark/network/util/NettyUtils.java
index caa7260bc8..10de9d3a5c 100644
--- a/common/network-common/src/main/java/org/apache/spark/network/util/NettyUtils.java
+++ b/common/network-common/src/main/java/org/apache/spark/network/util/NettyUtils.java
@@ -31,8 +31,6 @@ import io.netty.channel.epoll.EpollSocketChannel;
 import io.netty.channel.nio.NioEventLoopGroup;
 import io.netty.channel.socket.nio.NioServerSocketChannel;
 import io.netty.channel.socket.nio.NioSocketChannel;
-import io.netty.handler.codec.ByteToMessageDecoder;
-import io.netty.handler.codec.LengthFieldBasedFrameDecoder;
 import io.netty.util.internal.PlatformDependent;
 
 /**
diff --git a/common/network-common/src/main/java/org/apache/spark/network/util/TransportFrameDecoder.java b/common/network-common/src/main/java/org/apache/spark/network/util/TransportFrameDecoder.java
index a466c72915..3f7024a6aa 100644
--- a/common/network-common/src/main/java/org/apache/spark/network/util/TransportFrameDecoder.java
+++ b/common/network-common/src/main/java/org/apache/spark/network/util/TransportFrameDecoder.java
@@ -17,7 +17,6 @@
 
 package org.apache.spark.network.util;
 
-import java.util.Iterator;
 import java.util.LinkedList;
 
 import com.google.common.base.Preconditions;