Remove declaration of pyfb_isopen function

This function is removed because the framebuffer should be opened in a context that the application allready knows it is opened. It must be just like in OpenGL where the context is opened like in this case with open the framebuffer and closed. So each thread must know by itself if the framebuffer is opened or not.

native/pyframebuffer.h

@@ -233,15 +233,6 @@ extern void pyfb_close(uint8_t fbnum);
  */
 extern void pyfb_vinfo(uint8_t fbnum, struct pyfb_videomode_info* info_ptr);
 
-/**
- * Checks if the framebuffer number is opened.
- *
- * @param fbnum The framebuffer number to check
- *
- * @return If opened returns 0, else -1
- */
-extern int pyfb_isopen(uint8_t fbnum);
-
 /**
  * Paints a single pixel to the framebuffer. This function is secure because before
  * painting, it validates the arguments.