Move prober deletion action to backend module

Also creates a new package named 'batch' to house it.

TESTED=I deployed it to alpha, sent a POST request to the task URL, and it
successfully ran the [].

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=134332999

java/google/registry/env/sandbox/default/WEB-INF/cron.xml

@@ -98,8 +98,7 @@
     </description>
     <schedule>every monday 14:00</schedule>
     <timezone>UTC</timezone>
-    <!-- TODO(b/27309488): maybe move this to the backend module. -->
-    <target>tools</target>
+    <target>backend</target>
   </cron>
 
   <cron>