Remove Fl_Image::as_rgb_image() virtual member function.

It's not used in the library, so its utility remains to be proven.

FL/Fl_Image.H

@@ -213,12 +213,6 @@ public:
   // get RGB image scaling method
   static Fl_RGB_Scaling RGB_scaling();
   
-  /** Use this method if you have an Fl_Image object and want to know whether it is derived 
-   from class Fl_RGB_Image. 
-   If the method returns non-NULL, then the image in question is
-   derived from Fl_RGB_Image, and the returned value is a pointer to this image.
-   */
-  virtual Fl_RGB_Image *as_rgb_image() {return NULL;}
   // set the image drawing size
   virtual void scale(int width, int height, int proportional = 1, int can_expand = 0);
   /** Sets what algorithm is used when resizing a source image to draw it.
@@ -293,7 +287,6 @@ public:
    \sa  void Fl_RGB_Image::max_size(size_t)
    */
   static size_t max_size() {return max_size_;}
-  virtual Fl_RGB_Image *as_rgb_image() {return this;}
 };
 
 #endif // !Fl_Image_H