DRTVWR-418: Tweak shutdown sequence to avoid resurrecting singletons.

The LLSingletonBase::deleteAll() call late in LLAppViewer::cleanup() deletes
the LLSingleton(s) used by the logging machinery, among other things. Attempting
further logging after that call (such as our cheery "Goodbye!") has the
unfortunate effect of attempting to resurrect the deleted LLSingleton(s). Move
"Goodbye!" to just *before* the call.

Also, given that call, the manual references to a couple specific LLSingletons
in ~LLAppViewer() are (a) unnecessary and (b) cause attempted resurrection.
Eliminate both.