Remove previously deprecated headers, methods, and constants

Change-Id: I3e809e25c8f2bc2ae0f027413d9e0161245353c6
Refs: #3988
diff --git a/src/util/io.hpp b/src/util/io.hpp
index 7da812e..cad5a26 100644
--- a/src/util/io.hpp
+++ b/src/util/io.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /**
- * Copyright (c) 2013-2016 Regents of the University of California.
+ * Copyright (c) 2013-2017 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -62,8 +62,6 @@
   HEX
 };
 
-constexpr IoEncoding DEPRECATED(BASE_64) = BASE64;
-
 namespace detail {
 
 template<typename T>