VideoPlayer: Remove declarations for non-existent methods

Author: https://github.com/AtkinsSJ
Commit: 0cedbd2f50
Pull-request: https://github.com/SerenityOS/serenity/pull/17203
Reviewed-by: https://github.com/linusg ✅

Userland/Applications/VideoPlayer/VideoPlayerWidget.h

@@ -47,7 +47,6 @@ private:
     void set_time_label(Time);
     void on_decoding_error(Video::DecoderError const&);
     void update_seek_mode();
-    void display_next_frame();
 
     void cycle_sizing_modes();