[SPARK-16827] Stop reporting spill metrics as shuffle metrics

## What changes were proposed in this pull request?

Fix a bug where spill metrics were being reported as shuffle metrics. Eventually these spill metrics should be reported (SPARK-3577), but separate from shuffle metrics. The fix itself basically reverts the line to what it was in 1.6.

## How was this patch tested?

Tested on a job that was reporting shuffle writes even for the final stage, when no shuffle writes should take place. After the change the job no longer shows these writes.

Before:
![screen shot 2016-10-03 at 6 39 59 pm](https://cloud.githubusercontent.com/assets/1514239/19085897/dbf59a92-8a20-11e6-9f68-a978860c0d74.png)

After:
<img width="1052" alt="screen shot 2016-10-03 at 11 44 44 pm" src="https://cloud.githubusercontent.com/assets/1514239/19085903/e173a860-8a20-11e6-85e3-d47f9835f494.png">

Author: Brian Cho <bcho@fb.com>

Closes #15347 from dafrista/shuffle-metrics.

core/src/main/java/org/apache/spark/util/collection/unsafe/sort/UnsafeExternalSorter.java

@@ -145,7 +145,9 @@ public final class UnsafeExternalSorter extends MemoryConsumer {
     // Use getSizeAsKb (not bytes) to maintain backwards compatibility for units
     // this.fileBufferSizeBytes = (int) conf.getSizeAsKb("spark.shuffle.file.buffer", "32k") * 1024;
     this.fileBufferSizeBytes = 32 * 1024;
-    this.writeMetrics = taskContext.taskMetrics().shuffleWriteMetrics();
+    // The spill metrics are stored in a new ShuffleWriteMetrics, and then discarded (this fixes SPARK-16827).
+    // TODO: Instead, separate spill metrics should be stored and reported (tracked in SPARK-3577).
+    this.writeMetrics = new ShuffleWriteMetrics();
 
     if (existingInMemorySorter == null) {
       this.inMemSorter = new UnsafeInMemorySorter(