Clean up editorclass externs into one location

Again, like the previous commit, it should just be put in the header
file of its respective class instead of being a mess everywhere.

desktop_version/src/editor.cpp

@@ -2070,8 +2070,6 @@ void fillboxabs( int x, int y, int x2, int y2, int c )
 }
 
 
-extern editorclass ed;
-
 void editorclass::generatecustomminimap()
 {
     map.customwidth=mapwidth;