window-x11: Don't double-check for unmanaging windows

When looking for the best fallback focus window, we ignore it if it is in the
unmanaging state, but meta_stack_get_default_focus_window() does this is check
for us already.

So, ignore the redundant test.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/669


(cherry picked from commit 9aee47daa9)

src/x11/window-x11.c

@@ -852,9 +852,6 @@ meta_window_x11_focus (MetaWindow *window,
                       if (!focus_window)
                         break;
 
-                      if (focus_window->unmanaging)
-                        continue;
-
                       if (focus_window->input)
                         break;