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

drivers/md/bcache/stats.h

@@ -54,7 +54,6 @@ void bch_cache_accounting_destroy(struct cache_accounting *acc);
 
 void bch_mark_cache_accounting(struct cache_set *c, struct bcache_device *d,
 			       bool hit, bool bypass);
-void bch_mark_cache_readahead(struct cache_set *c, struct bcache_device *d);
 void bch_mark_cache_miss_collision(struct cache_set *c,
 				   struct bcache_device *d);
 void bch_mark_sectors_bypassed(struct cache_set *c,