Enable built-in session cleanup in production

[] enabled the built-in App Engine session cleanup servlet in alpha and sandbox, and it appears to be deleting expired sessions at the expected rate of 100 every 15 minutes. So enable it for production as well.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=138071390

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

@@ -220,5 +220,14 @@
     <target>backend</target>
   </cron>
 
+  <cron>
+    <url><![CDATA[/_ah/sessioncleanup?clear]]></url>
+    <description>
+      Delete up to 100 expired _ah_SESSION entities from Datastore.
+    </description>
+    <schedule>every 15 minutes</schedule>
+    <target>backend</target>
+  </cron>
+
 
 </cronentries>