Revert "io_uring: add zc notification flush requests"

This reverts commit 492dddb4.

Soon we won't have the very notion of notification flushing, so remove
notification flushing requests.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/8850334ca56e65b413cb34fd158db81d7b2865a3.1662027856.git.asml.silence@gmail.com


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