Remove CSSRule.UNKNOWN_RULE and CSSUnknownRule interface

Source/core/css/CSSUnknownRule.h

-/*
- * (C) 1999-2003 Lars Knoll (knoll@kde.org)
- * (C) 2002-2003 Dirk Mueller (mueller@kde.org)
- * Copyright (C) 2002, 2006, 2008, 2012 Apple Inc. All rights reserved.
- *
- * 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.
- */
-
-#ifndef CSSUnknownRule_h
-#define CSSUnknownRule_h
-
-#include "core/css/CSSRule.h"
-
-namespace blink {
-
-class CSSUnknownRule final : public CSSRule {
-    DEFINE_WRAPPERTYPEINFO();
-public:
-    CSSUnknownRule() : CSSRule(0) { }
-    virtual ~CSSUnknownRule() { }
-
-    virtual String cssText() const override { return String(); }
-    virtual void reattach(StyleRuleBase*) override { }
-    DEFINE_INLINE_VIRTUAL_TRACE() { CSSRule::trace(visitor); }
-
-private:
-    virtual CSSRule::Type type() const override { return UNKNOWN_RULE; }
-};
-
-} // namespace blink
-
-#endif // CSSUnknownRule_h