Revert "bump numparts up to 6"

This reverts commit daf96dd.

Author: holdenk

core/src/test/scala/org/apache/spark/storage/BlockManagerDecommissionIntegrationSuite.scala

@@ -34,7 +34,7 @@ class BlockManagerDecommissionIntegrationSuite extends SparkFunSuite with LocalS
     with ResetSystemProperties with Eventually {
 
   val numExecs = 3
-  val numParts = 6
+  val numParts = 3
 
   test(s"verify that an already running task which is going to cache data succeeds " +
     s"on a decommissioned executor") {