Cleanup GraphicsLayer by removing unused code

This patch removes the following unused functions from GraphicsLayer:
1) void addChildAtIndex(GraphicsLayer*, int index)
2) void addChildAbove(GraphicsLayer*, GraphicsLayer* sibling)
3) bool replaceChild(GraphicsLayer* oldChild, GraphicsLayer* newChild)

No new tests as there is no change in behavior.

Review URL: https://codereview.chromium.org/437353005

git-svn-id: svn://svn.chromium.org/blink/trunk@179531 bbb929c8-8fbe-4397-9dbb-9b2b20218538