ubd: remove dead code in ubd_setup_common

Remove some leftovers of the fake major number parsing that cause
complains from some compilers.

Fixes: 2933a1b2c6f3 ("ubd: remove the code to register as the legacy IDE driver")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210628093937.1325608-1-hch@lst.de


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