[SPARK-794][Core] Remove sleep() in ClusterScheduler.stop

Removed `sleep()` from the `stop()` method of the `TaskSchedulerImpl` class which, from the JIRA ticket, is believed to be a legacy artifact slowing down testing originally introduced in the `ClusterScheduler` class.

Author: Brennon York <brennon.york@capitalone.com>

Closes #3851 from brennonyork/SPARK-794 and squashes the following commits:

04c3e64 [Brennon York] Removed sleep() from the stop() method

core/src/main/scala/org/apache/spark/scheduler/TaskSchedulerImpl.scala

@@ -394,9 +394,6 @@ private[spark] class TaskSchedulerImpl(
       taskResultGetter.stop()
     }
     starvationTimer.cancel()
-
-    // sleeping for an arbitrary 1 seconds to ensure that messages are sent out.
-    Thread.sleep(1000L)
   }
 
   override def defaultParallelism() = backend.defaultParallelism()