Remove useless function Graphics::textboxcleanup()

It was made useless after my refactor to remove the 'active' system from
text boxes.

desktop_version/src/Graphics.h

@@ -55,8 +55,6 @@ public:
 
 	void createtextbox(std::string t, int xp, int yp, int r= 255, int g= 255, int b = 255);
 
-	void textboxcleanup();
-
 	void textboxcenter();
 
 	void textboxcenterx();