Revert "backend/native: Let get_seat_id() handle all modes"

This reverts commit a3082b8e.

We don't find the VKMS device with this commit because it is on seat0 and not on META_BACKEND_TEST_INPUT_SEAT.

The other way around, i.e. returning seat0 in all cases also doesn't work because something hangs if the default seat referrs to the real seat0 instead of the nonesense META_BACKEND_TEST_INPUT_SEAT.