[Java] fix java/cleanup.sh (#2989)

Remove legacy-ray-related stuff from this script, and update temp file locations.

java/cleanup.sh

@@ -1,10 +1,6 @@
-pkill -9 python
-pkill -9 local_scheduler
-pkill -9 plasma_manager
-pkill -9 plasma_store_server
-pkill -9 global_scheduler
-pkill -9 redis-server
-pkill -9 redis
-pkill -9 raylet
-ps aux | grep ray | awk '{system("kill "$2);}'
-rm /tmp/raylet*
+# Stop backend processes
+ray stop
+# Kill Java workers
+ps aux | grep DefaultWorker | grep -v grep | awk '{print $2}' | xargs kill -9
+# Remove temp files
+rm -rf /tmp/ray