io_uring: remove notif leftovers

Notifications were killed but there is a couple of fields and struct
declarations left, remove them.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/8df8877d677be5a2b43afd936d600e60105ea960.1664849941.git.asml.silence@gmail.com


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