io_uring: kill an outdated comment

Request referencing has changed a while ago and there is no notion left
of submission/completion references, kill an outdated comment.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/38902e7229d68cecd62702436d627d4858b0d9d4.1662639236.git.asml.silence@gmail.com


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