Don't zero opfuncid when reading nodes.

The comments here stated that this was just in case we ever had an
ALTER OPERATOR command that could remap an operator to a different
function.  But those comments have been here for a long time, and no
such command has come about.  In the absence of such a feature,
forcing the pg_proc OID to be looked up again each time we reread a
stored rule or similar is just a waste of cycles.  Moreover, parallel
query needs a way to reread the exact same node tree that was written
out, not one that has been slightly stomped on.  So just get rid of
this for now.

Per discussion with Tom Lane.

src/backend/nodes/readfuncs.c

@@ -665,17 +665,6 @@ _readOpExpr(void)
 
 	READ_OID_FIELD(opno);
 	READ_OID_FIELD(opfuncid);
-
-	/*
-	 * The opfuncid is stored in the textual format primarily for debugging
-	 * and documentation reasons.  We want to always read it as zero to force
-	 * it to be re-looked-up in the pg_operator entry.  This ensures that
-	 * stored rules don't have hidden dependencies on operators' functions.
-	 * (We don't currently support an ALTER OPERATOR command, but might
-	 * someday.)
-	 */
-	local_node->opfuncid = InvalidOid;
-
 	READ_OID_FIELD(opresulttype);
 	READ_BOOL_FIELD(opretset);
 	READ_OID_FIELD(opcollid);
@@ -696,17 +685,6 @@ _readDistinctExpr(void)
 
 	READ_OID_FIELD(opno);
 	READ_OID_FIELD(opfuncid);
-
-	/*
-	 * The opfuncid is stored in the textual format primarily for debugging
-	 * and documentation reasons.  We want to always read it as zero to force
-	 * it to be re-looked-up in the pg_operator entry.  This ensures that
-	 * stored rules don't have hidden dependencies on operators' functions.
-	 * (We don't currently support an ALTER OPERATOR command, but might
-	 * someday.)
-	 */
-	local_node->opfuncid = InvalidOid;
-
 	READ_OID_FIELD(opresulttype);
 	READ_BOOL_FIELD(opretset);
 	READ_OID_FIELD(opcollid);
@@ -727,17 +705,6 @@ _readNullIfExpr(void)
 
 	READ_OID_FIELD(opno);
 	READ_OID_FIELD(opfuncid);
-
-	/*
-	 * The opfuncid is stored in the textual format primarily for debugging
-	 * and documentation reasons.  We want to always read it as zero to force
-	 * it to be re-looked-up in the pg_operator entry.  This ensures that
-	 * stored rules don't have hidden dependencies on operators' functions.
-	 * (We don't currently support an ALTER OPERATOR command, but might
-	 * someday.)
-	 */
-	local_node->opfuncid = InvalidOid;
-
 	READ_OID_FIELD(opresulttype);
 	READ_BOOL_FIELD(opretset);
 	READ_OID_FIELD(opcollid);
@@ -758,17 +725,6 @@ _readScalarArrayOpExpr(void)
 
 	READ_OID_FIELD(opno);
 	READ_OID_FIELD(opfuncid);
-
-	/*
-	 * The opfuncid is stored in the textual format primarily for debugging
-	 * and documentation reasons.  We want to always read it as zero to force
-	 * it to be re-looked-up in the pg_operator entry.  This ensures that
-	 * stored rules don't have hidden dependencies on operators' functions.
-	 * (We don't currently support an ALTER OPERATOR command, but might
-	 * someday.)
-	 */
-	local_node->opfuncid = InvalidOid;
-
 	READ_BOOL_FIELD(useOr);
 	READ_OID_FIELD(inputcollid);
 	READ_NODE_FIELD(args);