block: remove two comments in bio_split_discard

A zero discard_granularity is not treated the same as a single-block one,
and not having any segments after taking alignment is perfectly fine
and does not need a warning.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20231228075545.362768-2-hch@lst.de


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