Implementing face clean up callback in forwarding strategy

Necessary for FwStats strategy that uses CcnxFace object as a key to
several statistics parameters.
diff --git a/model/forwarding-strategy/ccnx-forwarding-strategy.cc b/model/forwarding-strategy/ccnx-forwarding-strategy.cc
index 88451e6..ddc4e51 100644
--- a/model/forwarding-strategy/ccnx-forwarding-strategy.cc
+++ b/model/forwarding-strategy/ccnx-forwarding-strategy.cc
@@ -473,4 +473,11 @@
   // do nothing for now. may be need to do some logging
 }
 
+
+void
+CcnxForwardingStrategy::RemoveFace (Ptr<CcnxFace> face)
+{
+  // do nothing here
+}
+
 } //namespace ns3