DRTVWR-418: Use conventional LLSingleton init/cleanup for LLWinDebug.

LLWinDebug, though an LLSingleton, had (and required explicit calls to)
special init() and cleanup() methods. Kitty Barnett points out that the
cleanup() method was actually being called after LLSingletonBase::deleteAll(),
requiring resurrection of the deleted LLWinDebug, which sometimes led to
crashes. (Resurrecting deleted LLSingletons is always suspect.)

Change LLWinDebug::init() and cleanup() to the conventional initSingleton()
and cleanupSingleton() methods. This eliminates the need to make special
method calls at all. In particular, cleanupSingleton() will be called by the
existing LLSingletonBase::cleanupAll() call near viewer shutdown.

We retain the early LLWinDebug::instance() call, which implicitly initializes
the LLWinDebug instance, because evidently we want that initialized early. But
we no longer require a separate init() call.