Move tempstring off of Graphics, Game, and UtilityClass

Unlike, say, the scriptclass i/j/k stuff, these tempstrings are only
purely visual, and there's no known glitches to manipulate them anyway.
Thus I think it's safe to make this cleanup.

desktop_version/src/Graphics.h

@@ -198,8 +198,6 @@ public:
 
 	colourTransform ct;
 
-	std::string tempstring;
-
 	int bcol, bcol2, rcol;