Eliminate some offscreen surfaces in the scenario editor

- This fixes an issue where parts of the UI were filled with black or random noise.
- This also creates some noticeable lag while drawing, though.

src/scenedit/scen.graphics.hpp

@@ -4,7 +4,6 @@
 void Set_up_win ();
 void run_startup_g();
 void load_graphics();
-void load_main_screen();
 void draw_main_screen();
 void redraw_screen();
 void draw_lb();