Redundant code in create_gather_merge_path

In create_gather_merge_path, we should always guarantee that the
subpath is adequately ordered, and we do not add a Sort node in
createplan.c for a Gather Merge node. Therefore, the 'else' branch in
the snippet from create_gather_merge_path is redundant.

if (pathkeys_contained_in(pathkeys, subpath->pathkeys))
{
/* Subpath is adequately ordered, we won't need to sort it */
input_startup_cost += subpath->startup_cost;
input_total_cost += subpath->total_cost;
}
else
{
/* We'll need to insert a Sort node, so include cost for that */
Path sort_path; /* dummy for result of cost_sort */

cost_sort(&sort_path,
root,
pathkeys,
subpath->total_cost,
subpath->rows,
subpath->pathtarget->width,
0.0,
work_mem,
-1);
input_startup_cost += sort_path.startup_cost;
input_total_cost += sort_path.total_cost;
}

We should be able to assert that pathkeys_contained_in(pathkeys,
subpath->pathkeys) is always true, otherwise we'll be in trouble.

I noticed this while reviewing patch [1], thinking that it might be
worth fixing. Any thoughts?

[1] https://postgr.es/m/CAO6_Xqr9+51NxgO=XospEkUeAg-p=EjAWmtpdcZwjRgGKJ53iA@mail.gmail.com

Thanks
Richard