Remove gratuitous assumptions about what make_modifytable can see.

Remove gratuitous assumptions about what make_modifytable can see.

For no clearly good reason, make_modifytable assumed that it
could not reach its get-the-FDW-info-the-hard-way path in MERGE.
It's currently possible to demonstrate that assertion failing,
which seems to be due to an upstream planner bug; but there's no
good reason to do it like this at all.  Let's apply the principle
of separation of concerns and make the MERGE check separately,
after getting or not getting the fdwroutine pointer.

Per report from Alexander Lakhin.  No test case, since I think
the potential test condition will go away soon.

Discussion: https://postgr.es/m/36bee393-b351-16ac-93b2-d46d83637e45@gmail.com