Issues #197: Remove theme_radios (call theme_container instead).

PR for https://github.com/backdrop/backdrop-issues/issues/197
This should be `'#theme_wrappers' => array('container__radios'),` so that template/theme suggestions can target radios directly. Looks like this wasn't in the Drupal.org issue either, so I posted a comment over there that the same change needs to be made: https://drupal.org/node/2041825#comment-8881493