We don't use them[1], and these are generally not properties that
authors would be able to set via the font shorthand anyways.

Let's simplify the code. This fixes the font-variant bug and also
unblocks further clean-ups of these properties in the future.

[1]: https://searchfox.org/mozilla-central/rev/59f0bf3c13dd455d9f5415b89178de701ea6b850/widget/LookAndFeelTypes.ipdlh#12-18

Differential Revision: https://phabricator.services.mozilla.com/D160352