Fix misuse of foreach_delete_current().

Our coding convention requires this macro's result to be assigned
back to the original List variable.  In this usage, since the
List could not become empty, there was no actual bug --- but
some compilers warned about it.  Oversight in be45be9c3.

Discussion: https://postgr.es/m/35077b31-2d62-1e31-0e2e-ddb52d590b73@enterprisedb.com

src/backend/parser/parse_agg.c

@@ -1844,11 +1844,11 @@ expand_grouping_sets(List *groupingSets, bool groupDistinct, int limit)
 		list_sort(result, cmp_list_len_contents_asc);
 
 		/* Finally, remove duplicates */
-		prev = list_nth_node(List, result, 0);
+		prev = linitial_node(List, result);
 		for_each_from(cell, result, 1)
 		{
 			if (equal(lfirst(cell), prev))
-				foreach_delete_current(result, cell);
+				result = foreach_delete_current(result, cell);
 			else
 				prev = lfirst(cell);
 		}