Remove dead code from InsertionPoint

InsertionPoint::contains and indexOf are both unused so we can remove them.

R=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/19708005

git-svn-id: svn://svn.chromium.org/blink/trunk@154859 bbb929c8-8fbe-4397-9dbb-9b2b20218538