Notify that some evaluators may be removed.

def reset_evaluators(self, evaluators):
        """Notify that some evaluators may be removed."""
        for obj_id, ev in self._tasks.copy().items():
            if ev not in evaluators:
                del self._tasks[obj_id]
                del self._objects[obj_id]
        ok = []
        for ev, obj_id in self._fetching:
            if ev in evaluators:
                ok.append((ev, obj_id))
        self._fetching = ok