logger: create _vehicle_status_sub & _parameter_update_sub on stack

Since it's only used in run().

src/modules/logger/logger.h

@@ -128,8 +128,6 @@ private:
 
 	bool						_task_should_exit = true;
 	char 						_log_dir[64];
-	uORB::Subscription<vehicle_status_s>	*_vehicle_status_sub = nullptr;
-	uORB::Subscription<parameter_update_s>	*_parameter_update_sub = nullptr;
 	bool						_enabled = false;
 
 	// statistics