Remove unneeded vendor prefixes for box-shadow

[adamzap]

Browser versions that made these vendor prefixes obsolete were released by the middle of 2011: https://caniuse.com/css-boxshadow.

This is the first in what could be a long series of CSS/JS simplification PRs if this kind of thing is desired by the maintainers. I think we can reduce some complexity by taking small steps like this.

For instance, next I would want to factor the two affected mixins out. They are each used in one place.

[bmispelon]

This seems like a good change, thanks for submitting it 👍🏻

Thanks also for linking to caniuse, it makes reviewing easier.

In terms of further PRs, I'm looking forward to them and I think we could increase the scope a little bit. I checked with git grep -F -- '-moz-' '*.scss' and that yields about 20 results, which I think should be reviewable in a single PR (assuming all of these vendor prefixes can be removed, which might be incorrect).

I also wonder if there's a tool we could use to automatically flag these kinds of outdated CSS (having a linter we could run in CI would be amazing). I don't mind reviewing PRs like these one by one, but I'd prefer we could do it all in an automated single pass if that's possible.

[adamzap]

@bmispelon Ok thanks! I'll try to increase the scope while keeping different efforts separate, like removing vendor prefixes versus factoring Sass utilities out.