Since commit be8d8537 ("iothread: add I/O thread object") we
never used IOThreadClass / IOTHREAD_CLASS() / IOTHREAD_GET_CLASS(),
remove these definitions.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20230113200138.52869-2-philmd@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>