Handle `hidden_unless` for groups in `ListBuilder`

The `hidden_unless` option is supported by the list builder but is not
deleted from the options hash. It has to be deleted so it is not
displayed as an attribute of the html tag.

Without this pull request, the resulting HTML looks like
```html
<li class="has-nested" hidden_unless="#<Proc:0x007f9041fd0bf0@./config/initializers/navigasmic.rb:7>">
```