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.