[SPARK-15773][CORE][EXAMPLE] Avoid creating local variable `sc` in examples if possible

## What changes were proposed in this pull request?

Instead of using local variable `sc` like the following example, this PR uses `spark.sparkContext`. This makes examples more concise, and also fixes some misleading, i.e., creating SparkContext from SparkSession.
```
-    println("Creating SparkContext")
-    val sc = spark.sparkContext
-
     println("Writing local file to DFS")
     val dfsFilename = dfsDirPath + "/dfs_read_write_test"
-    val fileRDD = sc.parallelize(fileContents)
+    val fileRDD = spark.sparkContext.parallelize(fileContents)
```

This will change 12 files (+30 lines, -52 lines).

## How was this patch tested?

Manual.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #13520 from dongjoon-hyun/SPARK-15773.

2 files changed, 2 insertions, 6 deletions

diff --git a/examples/src/main/python/pi.py b/examples/src/main/python/pi.py
index b39d710540..e3f0c4aeef 100755
--- a/examples/src/main/python/pi.py
+++ b/examples/src/main/python/pi.py
@@ -32,8 +32,6 @@ if __name__ == "__main__":
         .appName("PythonPi")\
         .getOrCreate()
 
-    sc = spark.sparkContext
-
     partitions = int(sys.argv[1]) if len(sys.argv) > 1 else 2
     n = 100000 * partitions
 
@@ -42,7 +40,7 @@ if __name__ == "__main__":
         y = random() * 2 - 1
         return 1 if x ** 2 + y ** 2 < 1 else 0
 
-    count = sc.parallelize(range(1, n + 1), partitions).map(f).reduce(add)
+    count = spark.sparkContext.parallelize(range(1, n + 1), partitions).map(f).reduce(add)
     print("Pi is roughly %f" % (4.0 * count / n))
 
     spark.stop()
diff --git a/examples/src/main/python/transitive_closure.py b/examples/src/main/python/transitive_closure.py
index d88ea94e41..49551d4085 100755
--- a/examples/src/main/python/transitive_closure.py
+++ b/examples/src/main/python/transitive_closure.py
@@ -46,10 +46,8 @@ if __name__ == "__main__":
         .appName("PythonTransitiveClosure")\
         .getOrCreate()
 
-    sc = spark.sparkContext
-
     partitions = int(sys.argv[1]) if len(sys.argv) > 1 else 2
-    tc = sc.parallelize(generateGraph(), partitions).cache()
+    tc = spark.sparkContext.parallelize(generateGraph(), partitions).cache()
 
     # Linear transitive closure: each round grows paths by one edge,
     # by joining the graph's edges with the already-discovered paths.