Remove old DNS queue processing code.

The old DNS processing was performed by WriteDnsAction, which was invoked by the standard cron fanout action. The new code, which has been running for several months in production, uses ReadDnsQueueAction to do a custom fanout to PublishDnsUpdatesAction. We no longer need the old code, so it's time to remove it.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=127983115

javatests/google/registry/server/RegistryTestServer.java

@@ -73,7 +73,7 @@ public final class RegistryTestServer {
           google.registry.module.backend.BackendServlet.class),
 
       // Process DNS pull queue
-      route("/_dr/task/writeDns",
+      route("/_dr/cron/readDnsQueue",
           google.registry.module.backend.BackendServlet.class),
 
       // Registrar Console