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 [].

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

java/google/registry/env/common/backend/WEB-INF/web.xml

@@ -240,6 +240,12 @@
     <url-pattern>/_dr/task/exportDomainLists</url-pattern>
   </servlet-mapping>
 
+  <!--  Mapreduce to delete all prober data. -->
+  <servlet-mapping>
+    <servlet-name>backend-servlet</servlet-name>
+    <url-pattern>/_dr/task/deleteProberData</url-pattern>
+  </servlet-mapping>
+
   <!-- Deletes the specified contact resource if it is not referenced by any domains. -->
   <!-- TODO(b/26140521): Delete this mapping once non-batched async operations are deleted. -->
   <servlet-mapping>

java/google/registry/env/common/tools/WEB-INF/web.xml

@@ -66,13 +66,6 @@
     <url-pattern>/_dr/epptool</url-pattern>
   </servlet-mapping>
 
-  <!--  Mapreduce to delete all prober data. -->
-  <servlet-mapping>
-    <!-- TODO(b/27309488): maybe move this to the backend module + BackendServlet. -->
-    <servlet-name>tools-servlet</servlet-name>
-    <url-pattern>/_dr/task/deleteProberData</url-pattern>
-  </servlet-mapping>
-
   <!--  Mapreduce to re-save all EppResources. -->
   <servlet-mapping>
     <servlet-name>tools-servlet</servlet-name>