selftests: ublk: don't pass ${dev_id} to _cleanup_test()

More devices can be created in single tests, so simply remove all
ublk devices in _cleanup_test(), meantime remove the ${dev_id} argument
of _cleanup_test().

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250303124324.3563605-7-ming.lei@redhat.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>