blk-cgroup: Remove unused declaration blkg_path()

Commit bb7e5a19 ("block, bfq: remove blkg_path()") removed the
implementation but leave declaration.

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Link: https://lore.kernel.org/r/20240816095821.877842-1-yuehaibing@huawei.com


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