[sgen] Do GC handle clearing for domain unloading when world is stopped.

[sgen] Do GC handle clearing for domain unloading when world is stopped.

Previously we had two places where GC handles were cleared:

`mono_gchandle_free_domain()`, which was called with the world running.

`mono_gc_clear_domain()`, which ran with the world stopped.

The first one doesn't do anything anymore now.  The reason the callback is
still there is because that's where Boehm clears the GC handles.