3839 Hide Cancel Bookmark button from users for whom it does nothing

https://code.google.com/p/otwarchive/issues/detail?id=3839

On the top of works, the "Cancel Bookmark" button (link) was visible when you had JavaScript disabled. This was silly because it didn’t do anything. (It was also in a separate `<li>` from the "Bookmark" link, which was equally silly.)