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/Game.h

@@ -243,7 +243,6 @@ public:
 
     int mx, my;
     int screenshake, flashlight;
-    std::string tempstring;
     bool advancetext, pausescript;
 
     int deathseq, lifeseq;