[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.