Remove unused TextTrack virtual functions

Source/core/html/track/TextTrack.cpp

Index: Source/core/html/track/TextTrack.cpp
diff --git a/Source/core/html/track/TextTrack.cpp b/Source/core/html/track/TextTrack.cpp
index 0d26419b0b2dd4db71130083e4542e7cae254e15..f07ea38d87c8004e94e936b86e1ba9f64d6b31e6 100644
--- a/Source/core/html/track/TextTrack.cpp
+++ b/Source/core/html/track/TextTrack.cpp
@@ -475,15 +475,6 @@ bool TextTrack::hasCue(TextTrackCue* cue)
     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;