ublk: remove the empty open and release block device operations

No need to define empty versions, they can just be left out.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20220721130916.1869719-4-hch@lst.de


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