Flags a node as deleted, and removes it from the _edge_by_target (and parent's edge_by_source), if it is still found there.
        Does NOT remove the node's entries from self._edge_by_source.

def _flag_node_as_del_and_del_in_by_target(self, node_id):
        """Flags a node as deleted, and removes it from the _edge_by_target (and parent's edge_by_source), if it is still found there.
        Does NOT remove the node's entries from self._edge_by_source."""
        self.nodes_deleted.append(node_id)
        etp = self._edge_by_target.get(node_id)
        if etp is not None:
            del self._edge_by_target[node_id]