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>

Author: Christoph Hellwig

drivers/block/null_blk/main.c

@@ -1880,7 +1880,6 @@ static void null_config_discard(struct nullb *nullb)
 		return;
 	}
 
-	nullb->q->limits.discard_granularity = nullb->dev->blocksize;
 	blk_queue_max_discard_sectors(nullb->q, UINT_MAX >> 9);
 }