[css-values-5] Remove random(auto)

[fantasai]

Since the auto keyword for random() now is both the default behavior and also specifiable as property-index-scoped element-scoped, should we remove it? It's not, afaict, serving any purpose: the easiest way to specify this behavior, as well as the shortest serialization, is just to omit it.

[tabatkins]

It's still valuable to keep for random-item(), where you can't omit the caching key.

(I had the exact same thought until I remembered random-item().)