block: remove unused BIO_SPLIT_ENTRIES

Since commit 4b1faf93 ("block: Kill bio_pair_split()"), there's
no user of BIO_SPLIT_ENTRIES anymore.

Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>