style: Remove layout.css.webkit-appearance.enabled.
style: Remove layout.css.webkit-appearance.enabled.

I don't think we want to keep the ugly widget hacks forever. Let me know if
you'd rather keep the property behind a pref but I don't think there's a point
in doing that.

Differential Revision: https://phabricator.services.mozilla.com/D62649

KiChjang/servo,splav/servo,splav/servo,KiChjang/servo,splav/servo,KiChjang/servo,splav/servo,KiChjang/servo,KiChjang/servo,KiChjang/servo,splav/servo,splav/servo,splav/servo,KiChjang/servo,KiChjang/servo,KiChjang/servo,splav/servo,splav/servo,splav/servo,KiChjang/servo