Remove explicit interface versions of dartdoc files.

lib/html/doc/interface/SVGMaskElement.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 SVGMaskElement
-interface SVGMaskElement extends SVGElement, SVGTests, SVGLangSpace, SVGExternalResourcesRequired, SVGStylable {
-
-  /** @domName SVGMaskElement.height */
-  final SVGAnimatedLength height;
-
-  /** @domName SVGMaskElement.maskContentUnits */
-  final SVGAnimatedEnumeration maskContentUnits;
-
-  /** @domName SVGMaskElement.maskUnits */
-  final SVGAnimatedEnumeration maskUnits;
-
-  /** @domName SVGMaskElement.width */
-  final SVGAnimatedLength width;
-
-  /** @domName SVGMaskElement.x */
-  final SVGAnimatedLength x;
-
-  /** @domName SVGMaskElement.y */
-  final SVGAnimatedLength y;
-}