io_uring: kill outdated comment about overflow flush

__io_cqring_overflow_flush() doesn't return anything anymore, remove
outdate comment.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/4ce2bcbb17eac80cdf883fd1459d5ee6586e238c.1674484266.git.asml.silence@gmail.com


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