Temporarily revert phase two of migration away from ReferenceUnions

*** Reason for rollback ***

This code is fine, and I will be resurrecting it unaltered next week.  It's only being rolled back for operational reasons related to the timing of our internal pushes.

*** Original change description ***

Switch over to non-ReferenceUnion fields on DomainBase

This is the second phase of a three phase migration to remove
ReferenceUnions. As of the end of this phase, ReferenceUnions are no longer read
from in any active code paths, but are still written to in case a rollback to
the previous version is necessary. The third and final phase will remove the
ReferenceUnions entirely.

***

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=136642759

java/google/registry/model/EppResourceUtils.java

@@ -347,10 +347,12 @@ public final class EppResourceUtils {
   public static List<Key<DomainBase>> queryDomainsUsingResource(
       Class<? extends EppResource> clazz, Key<? extends EppResource> key, DateTime now, int limit) {
     checkArgument(ContactResource.class.equals(clazz) || HostResource.class.equals(clazz));
-    return ofy()
-        .load()
-        .type(DomainBase.class)
-        .filter(clazz.equals(ContactResource.class) ? "allContacts.contact" : "nsHosts", key)
+    return ofy().load().type(DomainBase.class)
+        .filter(
+            clazz.equals(ContactResource.class)
+                ? "allContacts.contactId.linked"
+                : "nameservers.linked",
+            key)
         .filter("deletionTime >", now)
         .limit(limit)
         .keys()