backends/native: Avoid meta_input_device_native_get_seat_impl()

These objects already have a pointer to the ClutterSeat that has a
pointer to the MetaSeatImpl in its native implementation. This data
may be considered pretty much immutable (a pointer to the seat is
held, and the native implementation will shut down the implementation
thread within ClutterSeat finalization.

Avoids some awkward code, since the MetaInputDeviceNative needs to
be aware of the Clutter object implementation and the implementation
object.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3278>

src/backends/native/meta-input-device-native.c

@@ -1563,12 +1563,6 @@ meta_input_device_native_new_virtual (MetaSeatImpl           *seat_impl,
   return CLUTTER_INPUT_DEVICE (device);
 }
 
-MetaSeatImpl *
-meta_input_device_native_get_seat_impl (MetaInputDeviceNative *device)
-{
-  return device->seat_impl;
-}
-
 void
 meta_input_device_native_update_leds_in_impl (MetaInputDeviceNative *device,
                                               enum libinput_led      leds)