Remove Traversal<Element> specializations for lastWithin() / previous()

Remove Traversal<Element> template specializations for lastWithin() /
previous() as they are now identical to the generic implementations after
r178540.

R=esprehn@chromium.org, abarth@chromium.org

Review URL: https://codereview.chromium.org/413133008

git-svn-id: svn://svn.chromium.org/blink/trunk@179064 bbb929c8-8fbe-4397-9dbb-9b2b20218538