Revert of Implement unset value handling

Source/core/css/CSSUnsetValue.h

-// Copyright 2014 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-#ifndef CSSUnsetValue_h
-#define CSSUnsetValue_h
-
-#include "core/css/CSSValue.h"
-#include "wtf/PassRefPtr.h"
-
-namespace blink {
-
-class CSSUnsetValue : public CSSValue {
-public:
-    static PassRefPtrWillBeRawPtr<CSSUnsetValue> create()
-    {
-        return adoptRefWillBeNoop(new CSSUnsetValue);
-    }
-
-    String customCSSText() const;
-
-    bool equals(const CSSUnsetValue&) const { return true; }
-
-    void traceAfterDispatch(Visitor* visitor) { CSSValue::traceAfterDispatch(visitor); }
-
-private:
-    CSSUnsetValue()
-        : CSSValue(UnsetClass)
-    {
-    }
-};
-
-DEFINE_CSS_VALUE_TYPE_CASTS(CSSUnsetValue, isUnsetValue());
-
-} // namespace blink
-
-#endif // CSSUnsetValue_h