Make metrics export run on backend module

This fixes a previous issue where metrics generated on the
tools/backend modules weren't being successfully exported to BigQuery.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=119047757

java/com/google/domain/registry/env/common/default/WEB-INF/web.xml

@@ -103,16 +103,6 @@
     <url-pattern>/check</url-pattern>
   </servlet-mapping>
 
-  <!-- Whitebox Metrics servlet. -->
-  <servlet>
-    <servlet-name>metrics</servlet-name>
-    <servlet-class>com.google.domain.registry.monitoring.whitebox.MetricsTaskServlet</servlet-class>
-  </servlet>
-  <servlet-mapping>
-    <servlet-name>metrics</servlet-name>
-    <url-pattern>/_dr/task/metrics</url-pattern>
-  </servlet-mapping>
-
   <!-- Security config -->
   <security-constraint>
     <web-resource-collection>