Remove warning remeving the method before redefining

We need to test if the same method defined more than once only register
one subscriber for it. We can safelly remove because the method body is
the same and Subscriber use method_added hook for register the
subscriber.

activesupport/test/subscriber_test.rb

@@ -23,6 +23,7 @@ def private_party(event)
 
 # Monkey patch subscriber to test that only one subscriber per method is added.
 class TestSubscriber
+  remove_method :open_party
   def open_party(event)
     events << event
   end