[youtube] Use a static algorithm for the age gate videos' signatures (fixes #3270)

I haven't managed to get the swf parser to work with the current swf player: http://s.ytimg.com/yts/swfbin/player-vflq0-BLs/watch_as3.swf (from http://www.youtube.com/watch?v=BaW_jenozKc), I always get an `unsupported opcode` message. Since the algorithm hasn't changed in a long time, I think it is reasonable to continue using it.
 
Pinging @phihag.