style: Remove :unsolved pseudo-class
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This is the servo part changes of https://bugzilla.mozilla.org/show_bug.cgi?id=1417829.
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix https://bugzilla.mozilla.org/show_bug.cgi?id=1417829.
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- [ ] There are tests for these changes OR
- [X] These changes do not require tests because this PR is to remove a feature.
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f47cd758fa
