io-wq: remove unused io-wq refcounting

iowq->refs is initialised to one and killed on exit, so it's not used
and we can kill it.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/401007393528ea7c102360e69a29b64498e15db2.1623634181.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>