ubd: remove the queue pointer in struct ubd

No need for it now, everything goes through the gendisk.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Richard Weinberger <richard@nod.at>
Link: https://lore.kernel.org/r/20240222072417.3773131-7-hch@lst.de


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