Revert "SPARK-2711. Create a ShuffleMemoryManager to track memory for all spilling collections"

This reverts commit 4fde28c2063f673ec7f51d514ba62a73321960a1.

Conflicts:
	core/src/main/scala/org/apache/spark/SparkEnv.scala
	core/src/main/scala/org/apache/spark/util/collection/ExternalSorter.scala
Revert #2