model+helper+examples: Enable configurability of NFD parameters using StackHelper

Refs: #2353, #2220
diff --git a/examples/ndn-tree-with-l2tracer.cpp b/examples/ndn-tree-with-l2tracer.cpp
index 52b8e8d..ab80675 100644
--- a/examples/ndn-tree-with-l2tracer.cpp
+++ b/examples/ndn-tree-with-l2tracer.cpp
@@ -38,8 +38,7 @@
   /****************************************************************************/
   // Install NDN stack on all nodes
   ndn::StackHelper ndnHelper;
-  ndnHelper.SetContentStoreChoice(false);
-  ndnHelper.SetContentStore("ns3::ndn::cs::Lru", "MaxSize", "1000");
+  ndnHelper.SetOldContentStore("ns3::ndn::cs::Lru", "MaxSize", "1000");
   ndnHelper.InstallAll();
   /****************************************************************************/
   // Installing global routing interface on all nodes