[SPARK-14308][ML][MLLIB] Remove unused mllib tree classes and move private classes to ML

## What changes were proposed in this pull request?

Decision tree helper classes will be migrated to ML. This patch moves those internal classes that are not part of the public API and removes ones that are no longer used, after [SPARK-12183](https://github.com/apache/spark/pull/11855). No functional changes are made.

Details:
* Bin.scala is removed as the ML implementation does not require bins
* mllib NodeIdCache is removed. It was only used by the mllib implementation previously, which no longer exists
* mllib TreePoint is removed. It was only used by the mllib implementation previously, which no longer exists
* BaggedPoint, DTStatsAggregator, DecisionTreeMetadata, BaggedPointSuite and TimeTracker are all moved to ML.

## How was this patch tested?

No functional changes are made. Existing unit tests ensure behavior is unchanged.

Author: sethah <seth.hendrickson16@gmail.com>

Closes #12097 from sethah/cleanup_mllib_tree.

3 files changed, 2 insertions, 3 deletions

diff --git a/mllib/src/test/scala/org/apache/spark/mllib/tree/impl/BaggedPointSuite.scala b/mllib/src/test/scala/org/apache/spark/ml/tree/impl/BaggedPointSuite.scala
index 9d756da410..77ab3d8bb7 100644
--- a/mllib/src/test/scala/org/apache/spark/mllib/tree/impl/BaggedPointSuite.scala
+++ b/mllib/src/test/scala/org/apache/spark/ml/tree/impl/BaggedPointSuite.scala
@@ -15,7 +15,7 @@
  * limitations under the License.
  */
 
-package org.apache.spark.mllib.tree.impl
+package org.apache.spark.ml.tree.impl
 
 import org.apache.spark.SparkFunSuite
 import org.apache.spark.mllib.tree.EnsembleTestHelper
diff --git a/mllib/src/test/scala/org/apache/spark/ml/tree/impl/RandomForestSuite.scala b/mllib/src/test/scala/org/apache/spark/ml/tree/impl/RandomForestSuite.scala
index 441338e74e..e64551f03c 100644
--- a/mllib/src/test/scala/org/apache/spark/ml/tree/impl/RandomForestSuite.scala
+++ b/mllib/src/test/scala/org/apache/spark/ml/tree/impl/RandomForestSuite.scala
@@ -26,7 +26,6 @@ import org.apache.spark.mllib.linalg.{Vector, Vectors}
 import org.apache.spark.mllib.regression.LabeledPoint
 import org.apache.spark.mllib.tree.{DecisionTreeSuite => OldDTSuite, EnsembleTestHelper}
 import org.apache.spark.mllib.tree.configuration.{Algo => OldAlgo, QuantileStrategy, Strategy => OldStrategy}
-import org.apache.spark.mllib.tree.impl.{BaggedPoint, DecisionTreeMetadata}
 import org.apache.spark.mllib.tree.impurity.{Entropy, Gini, GiniCalculator}
 import org.apache.spark.mllib.util.MLlibTestSparkContext
 import org.apache.spark.mllib.util.TestingUtils._
diff --git a/mllib/src/test/scala/org/apache/spark/mllib/tree/DecisionTreeSuite.scala b/mllib/src/test/scala/org/apache/spark/mllib/tree/DecisionTreeSuite.scala
index bb1041b109..49cb7e1f24 100644
--- a/mllib/src/test/scala/org/apache/spark/mllib/tree/DecisionTreeSuite.scala
+++ b/mllib/src/test/scala/org/apache/spark/mllib/tree/DecisionTreeSuite.scala
@@ -20,12 +20,12 @@ package org.apache.spark.mllib.tree
 import scala.collection.JavaConverters._
 
 import org.apache.spark.SparkFunSuite
+import org.apache.spark.ml.tree.impl.DecisionTreeMetadata
 import org.apache.spark.mllib.linalg.Vectors
 import org.apache.spark.mllib.regression.LabeledPoint
 import org.apache.spark.mllib.tree.configuration.Algo._
 import org.apache.spark.mllib.tree.configuration.FeatureType._
 import org.apache.spark.mllib.tree.configuration.Strategy
-import org.apache.spark.mllib.tree.impl.DecisionTreeMetadata
 import org.apache.spark.mllib.tree.impurity.{Entropy, Gini, Variance}
 import org.apache.spark.mllib.tree.model._
 import org.apache.spark.mllib.util.MLlibTestSparkContext