HostInterface: Move 'System shut down' message to SDL

Only frontend where it makes sense anyway.

src/core/host_interface.cpp

@@ -306,10 +306,7 @@ bool HostInterface::SaveState(const char* filename)
 
 void HostInterface::OnSystemCreated() {}
 
-void HostInterface::OnSystemDestroyed()
-{
-  ReportFormattedMessage("System shut down.");
-}
+void HostInterface::OnSystemDestroyed() {}
 
 void HostInterface::OnSystemPerformanceCountersUpdated() {}
 

src/duckstation-sdl/sdl_host_interface.cpp

@@ -314,6 +314,7 @@ void SDLHostInterface::OnSystemPaused(bool paused)
 void SDLHostInterface::OnSystemDestroyed()
 {
   CommonHostInterface::OnSystemDestroyed();
+  ReportFormattedMessage("System shut down.");
 }
 
 void SDLHostInterface::OnRunningGameChanged()