Remove "active_player_id" game state label (CT_CHEER)

[Whebon]

I assumed the reference to the active player gets lost when leaving an activeplayer state.
But that turns out to be false, so there is no need to store the "active_player_id" as a game state label.
Instead, to retrieve the last active player, I can just use $this->getActivePlayerId() in game/multipleactiveplayer states.