Remove TextTrackCue::isVTTCue

It is unused, as virtual functions are used instead of casting.

BUG=270340

Review URL: https://codereview.chromium.org/102003002

git-svn-id: svn://svn.chromium.org/blink/trunk@163113 bbb929c8-8fbe-4397-9dbb-9b2b20218538