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/utils/load-stats-node.cc b/utils/load-stats-node.cc
index e64bb9d..04ac660 100644
--- a/utils/load-stats-node.cc
+++ b/utils/load-stats-node.cc
@@ -170,6 +170,15 @@
   return zero;  
 }
 
+
+void
+LoadStatsNode::RemoveFace (ns3::Ptr<ns3::CcnxFace> face)
+{
+  NS_LOG_FUNCTION (this);
+  m_incoming.erase (face);
+  m_outgoing.erase (face);
+}
+
 bool
 LoadStatsNode::operator == (const LoadStatsNode &other) const
 {