io_uring/rw: remove leftover debug statement

This debug statement was never meant to go into the upstream release,
kill it off before it ends up in a release. It was just part of the
testing for the initial version of the patch.

Fixes: 2ec33a6c ("io_uring/rw: ensure kiocb_end_write() is always called")
Signed-off-by: Jens Axboe <axboe@kernel.dk>