DRTVWR-494: Move most LLSingleton cleanup back to destructor

instead of deleteSingleton().

Specifically, clear static SingletonData and remove the instance from the
MasterList in the destructor.

Empirically, some consumers are manually deleting LLSingleton instances,
instead of calling deleteSingleton(). If deleteSingleton() handles cleanup
rather than the destructor, we're left with dangling pointers in the Master
List.

We don't also call cleanupSingleton() from the destructor because only
deleteSingleton() promises to call cleanupSingleton(). Hopefully whoever is
directly deleting an LLSingleton subclass instance isn't relying on
cleanupSingleton().