Remove `clip` from our hide-visually() mixin

[MrDinsdale]

The clip CSS property which is currently used in our hide-visually() mixin (and potentially elsewhere) has been deprecated for some time. It currently still works in most browsers however will likely be removed at some point in the future.

We should look at alternatives, clip-path has poor support on legacy browsers but we might be able to utilise clip-path with clip as a fallback?

See https://developer.mozilla.org/en/docs/Web/CSS/clip

[joe-bell]

@MrDinsdale what do you propose for this?

Just cautious of the fact this has been open since last August!

[steallan-sky]

The article is a few years old but WebAIM have a slightly different recommendation for hiding text: https://webaim.org/techniques/css/invisiblecontent/ using text-indent: -10000px;.