[Spark-5889] Remove pid file after stopping service.

Currently the pid file is not deleted, and potentially may cause some problem after service is stopped. The fix remove the pid file after service stopped.

Author: Zhan Zhang <zhazhan@gmail.com>

Closes #4676 from zhzhan/spark-5889 and squashes the following commits:

eb01be1 [Zhan Zhang] solve review comments
b4c009e [Zhan Zhang] solve review comments
018110a [Zhan Zhang] spark-5889: remove pid file after stopping service
088d2a2 [Zhan Zhang] squash all commits
c1f1fa5 [Zhan Zhang] test

sbin/spark-daemon.sh

@@ -168,7 +168,7 @@ case $option in
       TARGET_ID="$(cat "$pid")"
       if [[ $(ps -p "$TARGET_ID" -o comm=) =~ "java" ]]; then
         echo "stopping $command"
-        kill "$TARGET_ID"
+        kill "$TARGET_ID" && rm -f "$pid"
       else
         echo "no $command to stop"
       fi