sm: Relocate the service manager to the System class

Rather than have it live as a global variable, place it into the System class and make it instance-based.

src/core/hle/service/sm/sm.cpp

@@ -67,7 +67,5 @@ ResultVal<Kernel::SharedPtr<Kernel::ClientSession>> ServiceManager::ConnectToSer
     return client_port->Connect();
 }
 
-std::shared_ptr<ServiceManager> g_service_manager;
-
 } // namespace SM
 } // namespace Service

src/core/hle/service/sm/sm.h

@@ -52,7 +52,5 @@ private:
     std::unordered_map<std::string, Kernel::SharedPtr<Kernel::ClientPort>> registered_services;
 };
 
-extern std::shared_ptr<ServiceManager> g_service_manager;
-
 } // namespace SM
 } // namespace Service