No need to add `disabled` class to disabled radio button.

[lcreid]

We add the disabled class to disabled radio buttons, but not check boxes. This shows that it doesn't seem to be needed. Having disabled="disabled" in the input is enough for Bootstrap 4 to format things correctly.

Removing the class would make the HTML more consistent across control types, and would make the code to implement the controls slightly less complex.

[lcreid]

Look at #156 before doing anything with this issue.

[lcreid]

https://getbootstrap.com/docs/4.0/components/forms/#disabled-forms confirms that for Bootstrap 4, the correct approach is to provide the disabled="disabled" attribute on the input tag. The class is no longer needed.