Remove now-unused function mapclass::RGB()

This function was only used in assigments to mapclass::towercol. But
that variable is unused, and has been removed, so after removing that
variable, this one is unused, too.

desktop_version/src/Map.h

@@ -22,8 +22,6 @@ class mapclass
 public:
     mapclass();
 
-    int RGB(int red,int green,int blue);
-
     int intpol(int a, int b, float c);
 
     void setteleporter(int x, int y);