Removed unnecessary friend declaration

Not needed as of 056d7ac8f8.

src/game_events/manager.hpp

@@ -89,9 +89,6 @@ private:
 		game_data* gamedata_;
 	};
 
-	// Performs an assertion check to ensure these members are not null.
-	friend void event_handler::disable();
-
 	const std::unique_ptr<event_handlers> event_handlers_;
 	std::set<std::string> unit_wml_ids_;