fix: remove cypher read clause restriction.

[emotionbug]

#589

It seems to be a transaction issue related to the removed logic now, but it needs to be checked.

agensgraph/src/backend/executor/nodeNestloop.c

Lines 174 to 204 in ee9b299

	if (node->js.jointype == JOIN_CYPHER_MERGE ||
	node->js.jointype == JOIN_CYPHER_DELETE)
	{
	svCid = innerPlan->state->es_snapshot->curcid;
	innerPlan->state->es_snapshot->curcid = node->nl_graphwrite_cid;
	}
	innerTupleSlot = ExecProcNode(innerPlan);
	econtext->ecxt_innertuple = innerTupleSlot;
	if (svCid != InvalidCommandId)
	innerPlan->state->es_snapshot->curcid = svCid;
	if (TupIsNull(innerTupleSlot))
	{
	ENL1_printf("no inner tuple, need new outer tuple");
	node->nl_NeedNewOuter = true;
	if (!node->nl_MatchedOuter &&
	(node->js.jointype == JOIN_LEFT ||
	node->js.jointype == JOIN_CYPHER_MERGE ||
	node->js.jointype == JOIN_CYPHER_DELETE ||
	node->js.jointype == JOIN_ANTI))
	{
	/*
	* We are doing an outer join and there were no join matches
	* for this outer tuple. Generate a fake join tuple with
	* nulls for the inner tuple, and return it if it passes the
	* non-join quals.
	*/