dm: remove legacy code only needed before submit_bio recursion

Commit 8615cb65 ("dm: remove useless loop in
__split_and_process_bio") showcased that we no longer loop.

Remove the bio_advance() in __split_and_process_bio() that was only
needed when looping was possible.

Similarly there is no need to advance the bio, using ci->sector
cursor, in __send_duplicate_bios().

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>