Remove explicit interface versions of dartdoc files.

lib/html/doc/interface/DocumentType.dartdoc

-// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
-// for details. All rights reserved. Use of this source code is governed by a
-// BSD-style license that can be found in the LICENSE file.
-
-// WARNING:
-//   This file contains documentation that is merged into the real source.
-//   Do not make code changes here.
-
-/// @domName DocumentType
-interface DocumentType extends Node {
-
-  /** @domName DocumentType.entities */
-  final NamedNodeMap entities;
-
-  /** @domName DocumentType.internalSubset */
-  final String internalSubset;
-
-  /** @domName DocumentType.name */
-  final String name;
-
-  /** @domName DocumentType.notations */
-  final NamedNodeMap notations;
-
-  /** @domName DocumentType.publicId */
-  final String publicId;
-
-  /** @domName DocumentType.systemId */
-  final String systemId;
-}