Don't adjust ressortgroupref in generate_setop_child_grouplist()

This is already done inside assignSortGroupRef(), therefore is
redundant.

Oversight from 66c0185a3.

Reported-by: Tom Lane
Discussion: https://postgr.es/m/3703023.1711654574@sss.pgh.pa.us

src/backend/optimizer/plan/planner.c

@@ -7959,7 +7959,6 @@ generate_setop_child_grouplist(SetOperationStmt *op, List *targetlist)
 
 		/* assign a tleSortGroupRef, or reuse the existing one */
 		sgc->tleSortGroupRef = assignSortGroupRef(tle, targetlist);
-		tle->ressortgroupref = sgc->tleSortGroupRef;
 	}
 	Assert(lg == NULL);
 	return grouplist;