refactor(button-group): cleaning up

Created by: ffoodd

Two changes:

1. the hover() mixin used to apply the exact same styles than :focus, :active, .active stack above — which doesn't use dedicated mixins… Removing mixin here seems a good thing since we can simplify selectors stack and reduce complexity a bit;
2. a stylelint check is disabled for useless qualifying selector: [type="radio"] and [type="checkbox"] obviously applies to input elements, and removing this element selector doesn't prevent selector to match.

Tried this locally without seeing any regression. Did I miss something?