remove bUpdatePlayers condition from GameState:UpdateSongPosition

3 screen classes were updating player SongPositions, ScreenGameplay,
ScreenHowToPlay, and ScreenEdit. All 3 screens stopped GameSoundManager
entirely from updating song position by using "SOUND->HandleSongTimer(
false );" to make GameSoundManager::Update return early.

If a change to allow player SongPositions to update normally is going to
be made, this is a bit more proper.