Remove nesting

At the beginning of this project we thought we were going to need nested checkbox support. This requirement has not materialised, so I'm pulling out the support for it. I wouldn't have bothered, except adding support means wrapping our checkboxes in list items to semantically associate them with their children. Doing this creates a lot of audio clutter for screen reader users, so if we aren't using it, it's better that it goes.


As discussed IRL could this be named something like `js-auto-height-inner` so its utility is clearer.