gtkwin: remove a redundant test in delete_window.

We never expect to be passed a NULL GtkFrontend pointer, and even if
we were, we'd have crashed several lines above this test.

It was benign, of course, but Coverity (which pointed it out) dislikes
this kind of thing on the basis that it's confusing - you ought to
either test it for NULL properly, or not at all - and I see its point.