[css-fonts] Remove `font-variant` descriptor example

[meyerweb]

In d3db869, the font-variant descriptor of the @font-face rule was dropped, resolving #2531.

Despite this, the latest draft still has an example (https://w3c.github.io/csswg-drafts/css-fonts-4/#ex-descriptor-only-that-rule) specifically about this descriptor. It should be removed.

[estelle]

#7863