Remove references to Objectify (#1846)

This is not a complete removal of ofy as we still a dependency on it
(GaeUserIdConverter). But this PR removed it from a lot of places where
it's no longer needed.

util/src/main/java/google/registry/util/DiffUtils.java

@@ -81,7 +81,7 @@ public final class DiffUtils {
           && aValue == null
           && bValue instanceof Collection
           && ((Collection<?>) bValue).isEmpty()) {
-        // Ignore a mismatch between Objectify's use of null to store empty collections and our
+        // Ignore a mismatch between the use of null to store empty collections and our
         // code's builder methods, which yield empty collections for the same fields.  This
         // prevents useless lines of the form "[null, []]" from appearing in diffs.
       } else {