remove unnecessary pure virtual function definition

This is overrided differently in playsingle, playmp, and replay
controllers, and never is it called abstractly, which is what
this virtual declaration would suggest.

src/play_controller.hpp

@@ -114,7 +114,6 @@ class play_controller : public controller_base, public events::observer, public
 
 	void maybe_do_init_side(bool is_replay = false, bool only_visual = false);
 	void do_init_side(bool is_replay = false, bool only_visual = false);
-	virtual void play_side() = 0;
 
 	virtual void force_end_turn() = 0;
 	virtual void force_end_level(LEVEL_RESULT res) = 0;