Replace drawrect with draw_rect

Turns out `graphics.drawrect` exists. Well, not anymore!
This was another function from before the renderer rewrite which tried
to draw a rectangle by using four filled rectangles. We can draw
outline rectangles properly now, so let's make sure everywhere does it!

desktop_version/src/Graphics.h

@@ -296,8 +296,6 @@ public:
 
     void drawmap(void);
 
-    void drawrect(int x, int y, int w, int h, int r, int g, int b);
-
     void drawtowermap(void);
 
     void drawtowerspikes(void);