Resolves#16821
This PR removes a special case in the `not-*` variant compound that
removed `:is(…)` if it was the only part of the inversed selector list.
While in-theory this makes sense, `:is(…)` accepts a _forgiving_
selector list where as `:not(…)` does not. See the [last point
here](https://developer.mozilla.org/en-US/docs/Web/CSS/:not#description).
This is an issue specifically in combinations with variants that have
selectors that are not supported by all browsers yet, for example
`:open`.
It seems to be the most expected to simply keep the `:is(…)` here in any
case.
## Test plan
- Ensured the repro form #16821 now also works in browsers that do not
support `:open` (Safari and Firefox at the time of writing this):
<img width="484" alt="Screenshot 2025-02-26 at 15 36 22"
src="https://github.com/user-attachments/assets/f3391693-895b-4e44-8566-95e2960ec4e3"
/>
e938c58778