Remove CSSRule.UNKNOWN_RULE and CSSUnknownRule interface

Source/core/css/CSSUnknownRule.idl

-/*
- * Copyright (C) 2006 Apple Computer, Inc.
- * Copyright (C) 2006 Samuel Weinig <sam.weinig@gmail.com>
- *
- * This library is free software; you can redistribute it and/or
- * modify it under the terms of the GNU Library General Public
- * License as published by the Free Software Foundation; either
- * version 2 of the License, or (at your option) any later version.
- *
- * This library is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
- * Library General Public License for more details.
- *
- * You should have received a copy of the GNU Library General Public License
- * along with this library; see the file COPYING.LIB.  If not, write to
- * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
- * Boston, MA 02110-1301, USA.
- */
-
-// The CSSUnknownRule interface was introduced in DOM Level 2 Style, but has
-// been removed in CSSOM:
-// http://www.w3.org/TR/DOM-Level-2-Style/css.html#CSS-CSSUnknownRule
-// http://dev.w3.org/csswg/cssom/
-//
-// TODO(philipj): Remove CSSUnknownRule.
-
-interface CSSUnknownRule : CSSRule {
-};