xephyr: Don't check for SeatId anymore

After a change for the xserver to automatically determine the seat
based on the XDG_SEAT variable, xephyr stopped working. This was
because of an old feature where xephyr used to handle evdev
directly. This was dropped some time ago, and now this check is
not needed
4 jobs for fixxephyr in 2 minutes and 38 seconds (queued for 2 seconds)