[SPARK-17316][CORE] Fix the 'ask' type parameter in 'removeExecutor'

## What changes were proposed in this pull request?

Fix the 'ask' type parameter in 'removeExecutor' to eliminate a lot of error logs `Cannot cast java.lang.Boolean to scala.runtime.Nothing$`

## How was this patch tested?

Jenkins

Author: Shixiong Zhu <shixiong@databricks.com>

Closes apache#14983 from zsxwing/SPARK-17316-3.

core/src/main/scala/org/apache/spark/scheduler/cluster/CoarseGrainedSchedulerBackend.scala

@@ -416,7 +416,7 @@ class CoarseGrainedSchedulerBackend(scheduler: TaskSchedulerImpl, val rpcEnv: Rp
    */
   protected def removeExecutor(executorId: String, reason: ExecutorLossReason): Unit = {
     // Only log the failure since we don't care about the result.
-    driverEndpoint.ask(RemoveExecutor(executorId, reason)).onFailure { case t =>
+    driverEndpoint.ask[Boolean](RemoveExecutor(executorId, reason)).onFailure { case t =>
       logError(t.getMessage, t)
     }(ThreadUtils.sameThread)
   }