Remove unused code

Author: pjt0620

openxc/controllers/base.py

@@ -174,9 +174,6 @@ def _response_is_multiframe(self, response):
             return True
         return False
 
-    def _return_frame(self, response):
-        return response['payload'][8:]
-        
     def _return_final(self, response):
         if response['frame'] == -1:
             return True