remove useless method

the subclass sets the body on the response object, so we don't need the
superclass doing it too

actionpack/lib/action_controller/metal/rack_delegation.rb

@@ -14,11 +14,6 @@ def build_with_env(env = {}) #:nodoc:
       end
     end
 
-    def response_body=(body)
-      response.body = body if response
-      super
-    end
-
     def reset_session
       @_request.reset_session
     end