Remove unused functions in LocalizedStrings.

Remove the following functions which are not called:
 - clickToExitFullScreenText
 - textTrackClosedCaptionsText
 - textTrackNoLabelText
 - textTrackOffText
 - textTrackSubtitlesText

BUG=298020
TEST=none; no behavior changes.
R=keishi@chromium.org

Review URL: https://codereview.chromium.org/25188003

git-svn-id: svn://svn.chromium.org/blink/trunk@158557 bbb929c8-8fbe-4397-9dbb-9b2b20218538