removed last of the NodeOutput dependencies

Since all Nodes have an output that provides all output values now, we now
never need to add an output from a Node derivative. These changes remove the
last of them and disable the ability to add more outputs from a derivative.

app/node/output/timeline/timeline.h

@@ -52,14 +52,14 @@ public:
 
   TrackList* track_list(TrackType type) const;
 
-  NodeOutput* length_output() const;
-
   const rational& timeline_length() const;
 
   const rational& timebase() const;
 
   void SetTimebase(const rational &timebase);
 
+  virtual void Retranslate() override;
+
 signals:
   void LengthChanged(const rational& length);
   void TimebaseChanged(const rational &timebase);
@@ -80,8 +80,6 @@ private:
 
   QVector<TrackOutput*> track_cache_;
 
-  NodeOutput* length_output_;
-
   rational length_;
 
   rational timebase_;