block: remove the fallback case in queue_dma_alignment

Now that all updates go through blk_validate_limits the default of 511
is set at initialization time.  Also remove the unused NULL check as
calling this helper on a NULL queue can't happen (and doesn't make
much sense to start with).

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: John Garry <john.g.garry@oracle.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://lore.kernel.org/r/20240626142637.300624-8-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>