commit c18a1e09(block: introduce bio_clone_bioset_partial()) introduced
bio_clone_bioset_partial() for raid1 write behind IO. Now the write behind is
rewritten by Ming. We don't need the API any more, so revert the commit.

Cc: Christoph Hellwig <hch@lst.de>
Reviewed-by: NJens Axboe <axboe@fb.com>
Reviewed-by: NMing Lei <tom.leiming@gmail.com>
Signed-off-by: NShaohua Li <shli@fb.com>