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/fw-stats.cc b/model/forwarding-strategy/fw-stats.cc
index 9d15152..cd3b744 100644
--- a/model/forwarding-strategy/fw-stats.cc
+++ b/model/forwarding-strategy/fw-stats.cc
@@ -193,6 +193,13 @@
     }
 }
 
+void
+FwStats::RemoveFace (Ptr<CcnxFace> face)
+{
+  m_stats.RemoveFace (face);
+
+  super::RemoveFace (face);
+}
 
 
 } // namespace ndnSIM