Reset the group's subscription to only contain topics subscribed by this consumer.

def reset_group_subscription(self):
        """Reset the group's subscription to only contain topics subscribed by this consumer."""
        if self._user_assignment:
            raise IllegalStateError(self._SUBSCRIPTION_EXCEPTION_MESSAGE)
        assert self.subscription is not None, 'Subscription required'
        self._group_subscription.intersection_update(self.subscription)