libretro: fix build for obsolete method.

src/libretro/FrameBufferLIBRETRO.hxx

@@ -139,13 +139,6 @@ class FrameBufferLIBRETRO : public FrameBuffer
     */
     bool setVideoMode(const string& title, const VideoMode& mode) override { return true; }
 
-    /**
-      This method is called to invalidate the contents of the entire
-      framebuffer (ie, mark the current content as invalid, and erase it on
-      the next drawing pass).
-    */
-    void invalidate() override { }
-
     /**
       This method is called to create a surface with the given attributes.