uORB::Subscription minor API cleanup

* the forceInit() method was combined with the existing subscribe()
 * delete unused last_update()

src/modules/mavlink/mavlink_orb_subscription.cpp

@@ -50,7 +50,7 @@ MavlinkOrbSubscription::is_published()
 	} else if (!published && _subscribe_from_beginning) {
 		// For some topics like vehicle_command_ack, we want to subscribe
 		// from the beginning in order not to miss or delay the first publish respective advertise.
-		return _sub.forceInit();
+		return _sub.subscribe();
 	}
 
 	return false;