remove unneeded sync_network call

this was added at some place in the past when games woudl end via a
game_end_exception, and is not needed anymore§

src/playsingle_controller.cpp

@@ -665,7 +665,6 @@ void playsingle_controller::play_ai_turn()
 		}
 	} catch(...) {
 		DBG_NG << "Caught exception playing ai turn: " << utils::get_unknown_exception_type();
-		turn_data_.sync_network();
 		throw;
 	}