Remove restriction to INSERT/UPDATE/DELETE foreign relations (#6202)

- While spiking on implementing PXF using FDW,
  we noticed that error messages for file_fdw
  were different from upstream error messages
  when updating a file_fdw. GPDB introduced a
  check in `setTargetTable` for foreign relations
  that might have not been cleaned up during
  merge. This PR removes the check in `setTargetTable`
  and fixes the expected output in file_fdw.
  Running make installcheck on file_fdw, now
  succeeds, as we are matching the upstream
  error message "updates aren't supported".
Authored-by: NFrancisco Guerrero <aguerrero@pivotal.io>

src/backend/parser/parse_clause.c

@@ -364,14 +364,6 @@ setTargetTable(ParseState *pstate, RangeVar *relation,
 				 errmsg("permission denied: \"%s\" is a system catalog",
 						 RelationGetRelationName(pstate->p_target_relation))));
 
-	/* special check for DML on foreign relations */
-	if(RelationIsForeign(pstate->p_target_relation))
-		ereport(ERROR,
-				(errcode(ERRCODE_WRONG_OBJECT_TYPE),
-				 errmsg("foreign tables are read only. cannot change \"%s\"",
-						 RelationGetRelationName(pstate->p_target_relation))));
-
-
 	/* special check for DML on external relations */
 	if(RelationIsExternal(pstate->p_target_relation))
 	{