Remove WebSetting::setFullScreenEnabled

It is no longer called from Chromium:
https://codereview.chromium.org/216233008/

BUG=344002

Review URL: https://codereview.chromium.org/217293002

git-svn-id: svn://svn.chromium.org/blink/trunk@170446 bbb929c8-8fbe-4397-9dbb-9b2b20218538