Remove a GPDB_92_MERGE_FIXME

Upstream postgres commit e2fa76d8 extracted `set_base_rel_sizes` out from
`set_base_rel_pathlists`, and path related information doesn't exist in
`set_base_rel_sizes`, as the FIXME indicated,
childrel->cheapest_total_path->rows will not be available.

-           parent_rows += cdbpath_rows(root, childrel->cheapest_total_path);
-           width_avg += cdbpath_rows(root, childrel->cheapest_total_path) * childrel->width;
+           parent_rows += childrel->rows;
+           parent_size += childrel->width * childrel->rows;

Co-authored-by: Alexandra Wang<lewang@pivotal.io>
Co-authored-by: Gang Xiong<gxiong@pivotal.io>

src/backend/optimizer/path/allpaths.c

@@ -688,10 +688,6 @@ set_append_rel_size(PlannerInfo *root, RelOptInfo *rel,
 
 		if (childrel->rows > 0)
 		{
-			/*
-			 * GPDB_92_MERGE_FIXME:
-			 * The path for childrel hasn't been created by here.
-			 */
 			parent_rows += childrel->rows;
 			parent_size += childrel->width * childrel->rows;