loop: remove a spurious clear of discard_alignment

loop: remove a spurious clear of discard_alignment

The loop driver never sets a discard_alignment, so it also doens't need
to clear it to zero.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Link: https://lore.kernel.org/r/20220418045314.360785-9-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>