Remove unused TextTrack virtual functions

isClosedCaptions(), containsOnlyForcedSubtitles(),
isMainProgramContent() and isEasyToRead() are unused.

WebInbandTextTrack::isClosedCaptions() is implemented in
Chromium (WebInbandTextTrackImpl) by switching on kind. Since it
doesn't use OVERRIDE, it will still compile and can be removed
after a Blink roll.

BUG=315257

Review URL: https://codereview.chromium.org/59913005

git-svn-id: svn://svn.chromium.org/blink/trunk@161398 bbb929c8-8fbe-4397-9dbb-9b2b20218538