bcache: remove unused bch_mark_cache_readahead function def in stats.h

This is a cleanup for commit 1616a4c2 ("bcache: remove bcache device
self-defined readahead")', currently no user for
bch_mark_cache_readahead() since that commit.

Signed-off-by: Lin Feng <linf@wangsu.com>
Signed-off-by: Coly Li <colyli@suse.de>
Link: https://lore.kernel.org/r/20220919161647.81238-3-colyli@suse.de


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