Haiku: Removed unused variable.

Found by Cppcheck.

src/main/haiku/SDL_BApp.h

@@ -385,7 +385,6 @@ private:
     /* Members */
     std::vector<SDL_Window*> _window_map; /* Keeps track of SDL_Windows by index-id */
 
-    display_mode *_saved_mode;
     BGLView      *_current_context;
 };