Enforce ARIA rule

### DESCRIPTION
We can find `slick-slides` that have `aria-hidden="true"` and `tabindex="-1"` attributes. This breaks the ARIA rules to have any tabindex value (indicating the element is focusable) whilst also having the hidden flag for an element.

Found with 'Axe' Chrome extension to flag accessibility issues
Docs: https://dequeuniversity.com/rules/axe/3.2/aria-hidden-focus?application=AxeChrome

Replicate here: https://codesandbox.io/s/ppwkk5l6xx

### CURRENT:
```

### EXPECTED:
```

**Solution**: tabindex attibute should be removed when `aria-hidden="true"`