io_uring/rw: drop -EOPNOTSUPP check in __io_complete_rw_common()

A recent change ensured that the necessary -EOPNOTSUPP -> -EAGAIN
transformation happens inline on both the reader and writer side,
and hence there's no need to check for both of these anymore on
the completion handler side.

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