Remove useless function Screen::ClearScreen()

This function does not clear the screen at all, it does absolutely
nothing.

desktop_version/src/Screen.h

@@ -12,7 +12,6 @@ public:
 	void GetWindowSize(int* x, int* y);
 
 	void UpdateScreen(SDL_Surface* buffer, SDL_Rect* rect);
-	void ClearScreen(int colour);
 	void FlipScreen();
 
 	const SDL_PixelFormat* GetFormat();