iothread: Remove unused IOThreadClass / IOTHREAD_CLASS

Since commit be8d853766 ("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>

iothread.c

@@ -25,10 +25,6 @@
 #include "qemu/rcu.h"
 #include "qemu/main-loop.h"
 
-typedef ObjectClass IOThreadClass;
-
-DECLARE_CLASS_CHECKERS(IOThreadClass, IOTHREAD,
-                       TYPE_IOTHREAD)
 
 #ifdef CONFIG_POSIX
 /* Benchmark results from 2016 on NVMe SSD drives show max polling times around