[css-counter-styles-3] Extended @counter-style containing *-symbols are valid

[cdoublev]

Fixes #9258. This PR removes this emphasized part:

If a @counter-style uses the extends system, it must not contain a symbols or additive-symbols descriptor, or else the @counter-style rule is invalid.

It was reported in #5717 and should have been fixed in ba2a8cb.

They all follow the resolution in #1682.

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ba2a8cb and this PR leave multiple requirements ("must [not] contain") without explicitly saying what should happen when they are not met. In my opinion, "otherwise, the @counter-style does not define a counter style (but is still a valid at-rule)" should be added.

[tabatkins]

In my opinion, "otherwise, the @counter-style does not define a counter style (but is still a valid at-rule)" should be added.

Yup, sounds good.