[CORE] Remove unreachable Heartbeat message from Worker

It doesn't look to me Heartbeat is sent to Worker from anyone.

Author: Kan Zhang <kzhang@apache.org>

Closes #6163 from kanzhang/deadwood and squashes the following commits:

56be118 [Kan Zhang] [core] Remove unreachable Heartbeat message from Worker

core/src/main/scala/org/apache/spark/deploy/worker/Worker.scala

@@ -324,9 +324,6 @@ private[worker] class Worker(
         map(e => new ExecutorDescription(e.appId, e.execId, e.cores, e.state))
       sender ! WorkerSchedulerStateResponse(workerId, execs.toList, drivers.keys.toSeq)
 
-    case Heartbeat =>
-      logInfo(s"Received heartbeat from driver ${sender.path}")
-
     case RegisterWorkerFailed(message) =>
       if (!registered) {
         logError("Worker registration failed: " + message)