gdk: remove the display->list_devices vfunc

gdk: remove the display->list_devices vfunc

gdk_display_list_devices is deprecated and all the backends
implement the same fallback by delegating to the device manager
and caching the list (caching it is needed since the method does
not transfer ownership of the container).
The compat code can be shared among all backends and we can
initialize the list lazily only in the case someone calls the
deprecated method.

https://bugzilla.gnome.org/show_bug.cgi?id=762891