Redundant code in create_gather_merge_path

[Richard Guo]

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=ejawmtpdczwjrggkj5...@mail.gmail.com

Thanks
Richard