[threadpool] Fix `MonoTests.runtime.unload-appdomain-on-shutdown.exe` (#4135)

[threadpool] Fix `MonoTests.runtime.unload-appdomain-on-shutdown.exe` (#4135)

* [threadpool] Increase and decrease refcount when removing domain

* [threadpool] Fix unload-appdomain-on-shutdown.exe

If the threadpool is not initialized, there is no need to initialize it, as no worker is going to start in the domain because mono_domain_is_unloading would return TRUE.