uORB::Subscription minor API cleanup

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

src/modules/uORB/uORBDeviceNode.hpp

@@ -196,14 +196,6 @@ public:
 	int get_priority() const { return _priority; }
 	void set_priority(uint8_t priority) { _priority = priority; }
 
-	/**
-	 * Copies the timestamp of the last update atomically.
-	 *
-	 * @return uint64_t
-	 *   Returns the timestamp of the most recent data.
-	 */
-	hrt_abstime last_update();
-
 	/**
 	 * Copies data and the corresponding generation
 	 * from a node to the buffer provided.