Remove obsolete classes:

Legacy workarounds for Visual Studio non thread-safe initialization
of function local objects with static storage duration are removed:

* Remove LeakChecked
* Remove StaticObject
* Remove SharedSingleton

src/ripple/app/main/Application.cpp

@@ -105,7 +105,6 @@ class ApplicationImp
     : public ApplicationImpBase
     , public beast::RootStoppable
     , public beast::DeadlineTimer::Listener
-    , public beast::LeakChecked <ApplicationImp>
     , public BasicApp
 {
 private: