Remove explicit interface versions of dartdoc files.

lib/html/doc/interface/OutputElement.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 HTMLOutputElement
-interface OutputElement extends Element default _Elements {
-
-  OutputElement();
-
-  /** @domName HTMLOutputElement.defaultValue */
-  String defaultValue;
-
-  /** @domName HTMLOutputElement.form */
-  final FormElement form;
-
-  /** @domName HTMLOutputElement.htmlFor */
-  DOMSettableTokenList htmlFor;
-
-  /** @domName HTMLOutputElement.labels */
-  final NodeList labels;
-
-  /** @domName HTMLOutputElement.name */
-  String name;
-
-  /** @domName HTMLOutputElement.type */
-  final String type;
-
-  /** @domName HTMLOutputElement.validationMessage */
-  final String validationMessage;
-
-  /** @domName HTMLOutputElement.validity */
-  final ValidityState validity;
-
-  /** @domName HTMLOutputElement.value */
-  String value;
-
-  /** @domName HTMLOutputElement.willValidate */
-  final bool willValidate;
-
-  /** @domName HTMLOutputElement.checkValidity */
-  bool checkValidity();
-
-  /** @domName HTMLOutputElement.setCustomValidity */
-  void setCustomValidity(String error);
-}