bcache: Remove dead references to cache_readaheads

The cache_readaheads stat counter is not used anymore and should be
removed.

Signed-off-by: Andrea Tomassetti <andrea.tomassetti-opensource@devo.com>
Signed-off-by: Coly Li <colyli@suse.de>
Link: https://lore.kernel.org/r/20230615121223.22502-4-colyli@suse.de


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