Currently, some class unloading logic, e.g. ClassLoaderDataGraph::purge, is placed in PSParallelCompact::post_compact, which causes `ClassUnloadingContext` to be located at the same level of various phases inside full-gc.

However, all class unloading logic can be grouped together at the end of marking phase. For example, G1CollectedHeap::unload_classes_and_code.