wayland: do not explicitly focus xdg_popup

The keyboard focus semantics for non-grabbing xdg_shell v6 popups is
pretty undefined.

Same applies for subsurfaces, but in practice, subsurfaces never receive
keyboard focus, so it makes sense to do the same for non-grabbing
popups.

https://bugzilla.gnome.org/show_bug.cgi?id=773210