Remove dead code in pg_dump.

Coverity quite reasonably complained that this check for fout==NULL
occurred after we'd already dereferenced fout.  However, the check
is just dead code since there is no code path by which CreateArchive
can return a null pointer.  Errors such as can't-open-that-file are
reported down inside CreateArchive, and control doesn't return.
So let's silence the warning by removing the dead code, rather than
continuing to pretend it does something.

Coverity didn't complain about this before 5b5fea2a, so back-patch
to 9.5 like that patch.