Registry should not be a BackupGroupRoot

In [] I made Registry a BGR in prep for each Registry being in
its own entity group. But we decided not to do that, and in []
mcilwain@ moved Registry under the cross-tld root. So there's no reason
for Registry to implement BackupGroupRoot anymore.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=120402417

javatests/com/google/domain/registry/tools/CreateTldCommandTest.java

@@ -66,8 +66,6 @@ public class CreateTldCommandTest extends CommandTestCase<CreateTldCommand> {
         .isEqualTo(Registry.DEFAULT_REDEMPTION_GRACE_PERIOD);
     assertThat(registry.getPendingDeleteLength()).isEqualTo(Registry.DEFAULT_PENDING_DELETE_LENGTH);
     assertThat(registry.getCreationTime()).isIn(Range.closed(before, after));
-    assertThat(registry.getUpdateAutoTimestamp().getTimestamp())
-        .isEqualTo(registry.getCreationTime());
   }
 
   @Test