io_uring: kill io_cqring_ev_posted() and __io_cq_unlock_post()

__io_cq_unlock_post() is identical to io_cq_unlock_post(), and
io_cqring_ev_posted() has a single caller so migth as well just inline
it there.

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