io_uring: remove io_uring_cqwait_reg_arg

A separate wait argument registration API was removed, also delete
leftover uapi definitions.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/143b6a53591badac23632d3e6fa3e5db4b342ee2.1731942445.git.asml.silence@gmail.com


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