[threadpool] Fix race on runtime shutdown (#4263)

[threadpool] Fix race on runtime shutdown (#4263)

We cannot free the threadpool, because there is a race on shutdown where a managed thread may request a new threadpool thread, but we already destroyed the threadpool. So to avoid a use-after-free, we simply do not free the threadpool, as we won't be able to access the threadpool anyway because the ref count will be 0

Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=51219