[css-fonts-4] Remove mentions of invalid `@font-face`

[cdoublev]

According to #1133 (comment) and CSS Fonts 4:

[...] @font-face rules require a font-family and src descriptor; if either of these are missing, the @font-face rule must not be considered when performing the font matching algorithm.

Basically, it remains valid but it does not declare a resource (CSSOM allows to update it and to become considered when performing the font matching algorithm).

But later it is defined as invalid:

Name: font-family

[...] It is required for the @font-face rule to be valid.

Name: src

[...] It is required for the @font-face rule to be valid.

I propose to remove these mentions.

[svgeesus]

CSSWG resolution from 2017