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/net/impl/RPCSub.cpp

@@ -30,7 +30,6 @@ namespace ripple {
 // Subscription object for JSON-RPC
 class RPCSubImp
     : public RPCSub
-    , public beast::LeakChecked <RPCSub>
 {
 public:
     RPCSubImp (InfoSub::Source& source, boost::asio::io_service& io_service,