[SPARK-25543][K8S] Print debug message iff execIdsRemovedInThisRound is not empty.

## What changes were proposed in this pull request?

Spurious logs like /sec.
2018-09-26 09:33:57 DEBUG ExecutorPodsLifecycleManager:58 - Removed executors with ids  from Spark that were either found to be deleted or non-existent in the cluster.
2018-09-26 09:33:58 DEBUG ExecutorPodsLifecycleManager:58 - Removed executors with ids  from Spark that were either found to be deleted or non-existent in the cluster.
2018-09-26 09:33:59 DEBUG ExecutorPodsLifecycleManager:58 - Removed executors with ids  from Spark that were either found to be deleted or non-existent in the cluster.
2018-09-26 09:34:00 DEBUG ExecutorPodsLifecycleManager:58 - Removed executors with ids  from Spark that were either found to be deleted or non-existent in the cluster.

The fix is easy, first check if there are any removed executors, before producing the log message.

## How was this patch tested?

Tested by manually deploying to a minikube cluster.

Closes apache#22565 from ScrapCodes/spark-25543/k8s/debug-log-spurious-warning.

Authored-by: Prashant Sharma <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: ScrapCodes

resource-managers/kubernetes/core/src/main/scala/org/apache/spark/scheduler/cluster/k8s/ExecutorPodsLifecycleManager.scala

@@ -100,8 +100,11 @@ private[spark] class ExecutorPodsLifecycleManager(
         }
       }
     }
-    logDebug(s"Removed executors with ids ${execIdsRemovedInThisRound.mkString(",")}" +
-      s" from Spark that were either found to be deleted or non-existent in the cluster.")
+
+    if (execIdsRemovedInThisRound.nonEmpty) {
+      logDebug(s"Removed executors with ids ${execIdsRemovedInThisRound.mkString(",")}" +
+        s" from Spark that were either found to be deleted or non-existent in the cluster.")
+    }
   }
 
   private def onFinalNonDeletedState(