make Ipv4L4Demux an NsUnknown object.

src/node/node.h

@@ -32,7 +32,6 @@
 namespace ns3 {
 
 class ApplicationList;
-class Ipv4L4Demux;
 class Ipv4;
 class Udp;
 class Arp;
@@ -73,7 +72,6 @@ public:
   // Each of these has a default behavior of returning a null capability
   // of the correct type if one exists, or the nil pointer if no
   // null capability exists.
-  virtual Ipv4L4Demux*     GetIpv4L4Demux() const;
   virtual ApplicationList* GetApplicationList() const;
   virtual Ipv4 *           GetIpv4 (void) const;
   virtual Udp *            GetUdp (void) const;