Remove dead code from childIndex optimization

Source/core/dom/ElementRareData.h

Index: Source/core/dom/ElementRareData.h
diff --git a/Source/core/dom/ElementRareData.h b/Source/core/dom/ElementRareData.h
index 213f906d7ec85b78b3512d56b0dfd6c22df0c0be..cfac411cdf7a11a154f307a361b6dec7143ff2f6 100644
--- a/Source/core/dom/ElementRareData.h
+++ b/Source/core/dom/ElementRareData.h
@@ -63,9 +63,6 @@ public:
         clearElementFlag(TabIndexWasSetExplicitly);
     }
 
-    unsigned childIndex() const { return m_childIndex; }
-    void setChildIndex(unsigned index) { m_childIndex = index; }
-
     CSSStyleDeclaration& ensureInlineCSSStyleDeclaration(Element* ownerElement);
 
     void clearShadow() { m_shadow = nullptr; }
@@ -124,7 +121,6 @@ public:
 
 private:
     short m_tabindex;
-    unsigned short m_childIndex;
 
     LayoutSize m_minimumSizeForResizing;
     IntSize m_savedLayerScrollOffset;
@@ -155,7 +151,6 @@ inline IntSize defaultMinimumSizeForResizing()
 inline ElementRareData::ElementRareData(RenderObject* renderer)
     : NodeRareData(renderer)
     , m_tabindex(0)
-    , m_childIndex(0)
     , m_minimumSizeForResizing(defaultMinimumSizeForResizing())
 {
 }
@@ -220,7 +215,6 @@ inline PseudoElement* ElementRareData::pseudoElement(PseudoId pseudoId) const
 inline void ElementRareData::resetStyleState()
 {
     clearElementFlag(StyleAffectedByEmpty);
-    setChildIndex(0);
 }
 
 } // namespace