Remove polyfill for focus-visible

[wenche]

Background: We use a polyfill for focus-visible in the same way as EDS. The focus-visible is a way to solve the problem of adding a visible outline only if you use the keyboard for navigating and not when you click with the mouse.

This feature has a rather good support by the browsers now and we should look into to remove it completely and use the browser native solution instead.

[BorghildSelle]

Will be done in #2150