Remove explicit destructor from playsingle_controller

Now when the AI manager is part of game_state, destroying game_state
implicitly removes all observers from AI manager.
Therefore playsingle_controller no longer needs to explicitly deregister
itself as an observer.

src/playsingle_controller.cpp

@@ -106,12 +106,6 @@ std::string playsingle_controller::describe_result() const
 	}
 }
 
-playsingle_controller::~playsingle_controller()
-{
-	ai::manager::get_singleton().remove_observer(this);
-	ai::manager::get_singleton().clear_ais();
-}
-
 void playsingle_controller::init_gui()
 {
 	LOG_NG << "Initializing GUI... " << (SDL_GetTicks() - ticks()) << "\n";

src/playsingle_controller.hpp

@@ -36,7 +36,6 @@ class playsingle_controller : public play_controller
 public:
 	playsingle_controller(const config& level, saved_game& state_of_game,
 		const config& game_config, const ter_data_cache & tdata, bool skip_replay);
-	virtual ~playsingle_controller();
 
 	LEVEL_RESULT play_scenario(const config& level);
 	void play_scenario_init();