face: introduce NullTransport and use it in several places

Refs: #4528
Change-Id: Icc7a77232bca1a700ce1319493c15a85d05d7488
diff --git a/tests/daemon/face/dummy-face.hpp b/tests/daemon/face/dummy-face.hpp
index e4c68b3..93bcd48 100644
--- a/tests/daemon/face/dummy-face.hpp
+++ b/tests/daemon/face/dummy-face.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
-/**
- * Copyright (c) 2014-2016,  Regents of the University of California,
+/*
+ * Copyright (c) 2014-2019,  Regents of the University of California,
  *                           Arizona Board of Regents,
  *                           Colorado State University,
  *                           University Pierre & Marie Curie, Sorbonne University,
@@ -32,8 +32,6 @@
 namespace face {
 namespace tests {
 
-class DummyTransport;
-
 /** \brief a Face for unit testing
  *
  *  The DummyFace has no underlying transport, but allows observing outgoing packets
@@ -87,9 +85,6 @@
   LinkService*
   getLinkServiceInternal();
 
-  DummyTransport*
-  getTransportInternal();
-
 public:
   std::vector<Interest>& sentInterests;
   std::vector<Data>& sentData;
@@ -102,6 +97,7 @@
 namespace tests {
 using nfd::face::tests::DummyFace;
 } // namespace tests
+
 } // namespace nfd
 
 #endif // NFD_TESTS_DAEMON_FACE_DUMMY_FACE_HPP