3895 Remove RTE's option for adding target=_blank to links

https://code.google.com/p/otwarchive/issues/detail?id=3895

We don't allow the target attribute on links, so the RTE should not let you set Target to "New Window." This puts an empty array in for `target_list` and now the only menu item will be "None." http://www.tinymce.com/wiki.php/Configuration:target_list