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/track/track.h

@@ -52,8 +52,6 @@ public:
 
   NodeInput* track_input();
 
-  NodeOutput* track_output();
-
   Block* BlockContainingTime(const rational& time) const;
 
   Block* NearestBlockBefore(const rational& time) const;
@@ -160,8 +158,6 @@ private:
 
   NodeInput* track_input_;
 
-  NodeOutput* track_output_;
-
   TrackType track_type_;
 
   rational track_length_;