Remove a bunch of dead code and clean up more.

main.cpp

@@ -25,19 +25,16 @@ int main(int argc, char *argv[])
   textures::init();
 
   GameEngine game{100};
-  auto backend = SFMLBackend(game);
-  GUI gui(backend, timer_seconds);
+  GUI gui(timer_seconds);
   auto builder = Builder(gui, game);
 
-  backend.startup();
+  gui.startup();
 
-  while(backend.is_open()) {
+  while(gui.is_open()) {
     builder.event(BuildEvent::GO);
     gui.main_loop();
-    backend.window.display();
   }
 
   builder.event(BuildEvent::QUIT);
-  backend.shutdown();
   return 0;
 }