Switch pattern for marking unused argument

Author: gnossen

src/python/grpcio/grpc/_interceptor.py

@@ -126,8 +126,7 @@ def exception(self, ignored_timeout=None):
     def traceback(self, ignored_timeout=None):
         return self._traceback
 
-    def add_callback(self, callback):
-        del callback
+    def add_callback(self, unused_callback):
         return False
 
     def add_done_callback(self, fn):