Make static "Destroy" methods for certain singletons unnecessary.

src/AudacityApp.cpp

@@ -1984,10 +1984,6 @@ int AudacityApp::OnExit()
    // Terminate the PluginManager (must be done before deleting the locale)
    PluginManager::Get().Terminate();
 
-   // Done with plugins and modules
-   PluginManager::Destroy();
-   ModuleManager::Destroy();
-
    if (mLocale)
       delete mLocale;