Remove vendor prefixes from sass

[jnoordsij]

This is a new feature | improvement | bugfix | documentation fix.

Proposed solution

When debugging some other issues with placeholders in my application, I noticed some vendor prefixes popping up on attributes where I did not expect them. Then I figured that they are actually hardcoded within the scope of this repository, rather than applied by tooling (e.g. autoprefixer).

Edit: for the ::placeholder mixin, it was actually rendering all the variants except for the correct modern ::placeholder pseudo-element, so this actually might ensure proper behavior across environments that previously did not work.

This PR removes the hardcoded vendor-prefixed variants of some properties. The preferred way (I think) should be using tooling to handle this. Although the README mentions that autoprefixer is used within this repository, it seems it has since been replaced with cssnano, which may not include it by default.

Probably this requires rebuilding css too, however the npm run build alias mentioned in CONTRIBUTING.md no longer exists, so I'm not sure which parts are intended to run on MR submissions.

Note: I encountered this issue on version v0.9.4, but I'm not sure if bugfixes are still welcome for the old version?

Tradeoffs

Theoretically this could cause a change of behavior on legacy browsers. Given the wide availability of the non-prefixed alternatives and the fact that tooling like autoprefixer is commonly used, I suspect this will have little to no impact in that regard.

Testing Done

None.

Changelog updated?

No.