io_uring: kill unused notif declarations

There are two leftover structures from the notification registration
mechanism that has never been released, kill them.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/f05f65aebaf8b1b5bf28519a8fdb350e3e7c9ad0.1679924536.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>