[SPARK-9552] Return "false" while nothing to kill in killExecutors

In discussion (SPARK-9552), we proposed a force kill in `killExecutors`. But if there is nothing to kill, it will return back with true (acknowledgement). And then, it causes the certain executor(s) (which is not eligible to kill) adding to pendingToRemove list for further actions.

In this patch, we'd like to change the return semantics. If there is nothing to kill, we will return "false". and therefore  all those non-eligible executors won't be added to the pendingToRemove list.

vanzin andrewor14 As the follow up of PR#7888, please let me know your comments.

Author: Grace <jie.huang@intel.com>
Author: Jie Huang <hjie@fosun.com>
Author: Andrew Or <andrew@databricks.com>

Closes #9796 from GraceH/emptyPendingToRemove.