Remove unused TextTrack virtual functions

Source/core/html/track/TextTrack.cpp

 /*
  * Copyright (C) 2011 Google Inc.  All rights reserved.
  * Copyright (C) 2011, 2012, 2013 Apple Inc.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 457 matching lines @@
         if (cue->startTime() < existingCue->startTime() || (cue->startTime() == existingCue->startTime() && cue->endTime() > existingCue->endTime()))
             searchEnd = index;
         else
             searchStart = index + 1;
     }
 
     ASSERT_NOT_REACHED();
     return false;
 }
 
-bool TextTrack::isMainProgramContent() const
-{
-    // "Main program" content is intrinsic to the presentation of the media file, regardless of locale. Content such as
-    // directors commentary is not "main program" because it is not essential for the presentation. HTML5 doesn't have
-    // a way to express this in a machine-reable form, it is typically done with the track label, so we assume that caption
-    // tracks are main content and all other track types are not.
-    return m_kind == captionsKeyword();
-}
-
 const AtomicString& TextTrack::interfaceName() const
 {
     return EventTargetNames::TextTrack;
 }
 
 ExecutionContext* TextTrack::executionContext() const
 {
     return m_document;
 }
 
 } // namespace WebCore