Remove exports of removed notification functions

This API did not make it into the kernel release, and we dropped support
for it in liburing. It was never in a released liburing, but we forgot
to drop it from the exports list when the helpers were removed.

Fixes: 880a932c8ae3 ("zc: adjust sendzc to the simpler uapi")
Signed-off-by: Jens Axboe <axboe@kernel.dk>