null_blk: use the default discard granularity

The discard granularity now defaults to a single sector, so don't set
that value explicitly.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20231228075545.362768-7-hch@lst.de


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