io_uring: remove __io_fill_cqe() helper

In preparation for the following patch, inline __io_fill_cqe(), there is
only one user.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/71dab9afc3cde3f8b64d26f20d3b60bdc40726ff.1655287457.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>