Add @Deprecated to remove a warning

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=147478560

javatests/google/registry/testing/FakeUserService.java

@@ -42,6 +42,7 @@ public class FakeUserService implements UserService {
     throw new UnsupportedOperationException();
   }
 
+  @Deprecated
   @Override
   public String createLoginURL(String destinationURL, String authDomain, String federatedIdentity,
       Set<String> attributesRequest) {