Remove explicit interface versions of dartdoc files.

lib/html/doc/interface/MetaElement.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 HTMLMetaElement
-interface MetaElement extends Element {
-
-  /** @domName HTMLMetaElement.content */
-  String content;
-
-  /** @domName HTMLMetaElement.httpEquiv */
-  String httpEquiv;
-
-  /** @domName HTMLMetaElement.name */
-  String name;
-
-  /** @domName HTMLMetaElement.scheme */
-  String scheme;
-}