Drop generic concurrent rendering. It makes too many things very

complicated, and I don't trust it at a high level: it makes too
many unchecked things concurrent, so I think it will make things
brittle.

stepmania/src/StepMania.cpp

@@ -284,12 +284,6 @@ void ShutdownGame()
 
 static void HandleException( const RString &sError )
 {
-	if( SCREENMAN && SCREENMAN->IsConcurrentlyLoading() )
-	{
-		LOG->Trace( "HandleException: FinishConcurrentRendering" );
-		GameLoop::FinishConcurrentRendering();
-	}
-
 	if( g_bAutoRestart )
 		HOOKS->RestartProgram();