LibVideo: Remove declarations for non-existent methods

Author: https://github.com/AtkinsSJ
Commit: 5cc4b37bf3
Pull-request: https://github.com/SerenityOS/serenity/pull/17203
Reviewed-by: https://github.com/linusg ✅

Userland/Libraries/LibVideo/Containers/Matroska/Reader.h

@@ -97,7 +97,6 @@ private:
     {
     }
 
-    DecoderErrorOr<void> set_position(size_t position);
     DecoderErrorOr<void> seek_to_cue_point(CuePoint const& cue_point);
 
     RefPtr<Core::MappedFile> m_file;