Remove no longer used methods nextAncestorSibling() from NodeTraversal.h

These methods are no longer used and should be removed.  They were previously
used by the traverseNextTemplate methods that have since been merged into
wtf/TreeNode.h.

BUG=331880

Review URL: https://codereview.chromium.org/126123004

git-svn-id: svn://svn.chromium.org/blink/trunk@164639 bbb929c8-8fbe-4397-9dbb-9b2b20218538