bcache: remove unused bch_passthrough_cache

struct kmem_cache *bch_passthrough_cache is not used in
bcache code. Remove it.
Signed-off-by: Shenghui Wang <shhuiw@foxmail.com>
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/request.h

@@ -39,6 +39,6 @@ void bch_data_insert(struct closure *cl);
 void bch_cached_dev_request_init(struct cached_dev *dc);
 void bch_flash_dev_request_init(struct bcache_device *d);
 
-extern struct kmem_cache *bch_search_cache, *bch_passthrough_cache;
+extern struct kmem_cache *bch_search_cache;
 
 #endif /* _BCACHE_REQUEST_H_ */