[SPARK-4102] Remove unused ShuffleReader.stop() method.

This method is not implemented by the only subclass
(HashShuffleReader), nor is it ever called. While the
use of Scala's fancy "???" was pretty exciting, the method's
existence can only lead to confusion and it therefore should
be deleted.

mateiz was there a reason for adding this that I'm
missing?

Author: Kay Ousterhout <kayousterhout@gmail.com>

Closes #2966 from kayousterhout/SPARK-4102 and squashes the following commits:

532c564 [Kay Ousterhout] Added back commented-out method, as per Matei's request
904655e [Kay Ousterhout] [SPARK-4102] Remove unused ShuffleReader.stop() method.

core/src/main/scala/org/apache/spark/shuffle/ShuffleReader.scala

@@ -24,6 +24,10 @@ private[spark] trait ShuffleReader[K, C] {
   /** Read the combined key-values for this reduce task */
   def read(): Iterator[Product2[K, C]]
 
-  /** Close this reader */
-  def stop(): Unit
+  /**
+   * Close this reader.
+   * TODO: Add this back when we make the ShuffleReader a developer API that others can implement
+   * (at which point this will likely be necessary).
+   */
+  // def stop(): Unit
 }

core/src/main/scala/org/apache/spark/shuffle/hash/HashShuffleReader.scala

@@ -66,7 +66,4 @@ private[spark] class HashShuffleReader[K, C](
         aggregatedIter
     }
   }
-
-  /** Close this reader */
-  override def stop(): Unit = ???
 }