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=137951076

java/google/registry/model/domain/DesignatedContact.java

@@ -73,6 +73,6 @@ public class DesignatedContact extends ImmutableObject {
   }
 
   public Key<ContactResource> getContactKey() {
-    return contactId.getLinked();
+    return contact;
   }
 }