code: def drop_callback_reference(self, subscription):
          
          if subscription not in self.__subscriptions:
              raise workflows.Error(
                  "Attempting to drop callback reference for unknown subscription"
              )
          if not self.__subscriptions[subscription]["unsubscribed"]:
              raise workflows.Error(
                  "Attempting to drop callback reference for live subscription"
              )
          del self.__subscriptions[subscription]
docs: Drop reference to the callback function after unsubscribing.
          Any future messages arriving for that subscription will result in
          exceptions being raised.
          :param subscription: Subscription ID to delete callback reference for.