bcache: remove useless alloc_bucket_pages()

Now no one uses alloc_bucket_pages() anymore, remove it from bcache.h.

Signed-off-by: Coly Li <colyli@suse.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>