Remove webkitAllowFullScreen and mozallowfullscreen from block_video

These attributes are already deprecated and not needed since [Firefox 18](https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/18?redirectlocale=en-US&redirectslug=Firefox_18_for_developers#HTML) and [Chrome/Chromium 27](https://bugs.webkit.org/show_bug.cgi?id=110374).