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/LoadManager.cpp

@@ -28,6 +28,7 @@
 #include <beast/threads/Thread.h>
 #include <beast/cxx14/memory.h> // <memory>
 #include <mutex>
+#include <thread>
 
 namespace ripple {