io_uring: kill fasync

We have never supported fasync properly, it would only fire when there
is something polling io_uring making it useless. The original support came
in through the initial io_uring merge for 5.1. Since it's broken and
nobody has reported it, get rid of the fasync bits.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/2f7ca3d344d406d34fa6713824198915c41cea86.1633080236.git.asml.silence@gmail.com


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