Drop `hint-mode`'s Shadow DOM support

[aadcg]

Description

Fixes #3254

After a detailed examination, there is no fast way to re-write the feature in a timely fashion given the current priorities. On the other hand, the performance penalty introduced by the feature is rather severe and it is hurting the users. Issue #3295 hints at how it could be implemented.

Checklist:

• [x] Git branch state is mergable.
• [x] Changelog is up to date (via a separate commit).
• [x] New dependencies are accounted for.
• [x] Documentation is up to date.
• [x] Compilation and tests ((asdf:test-system :nyxt/gi-gtk))
  • No new compilation warnings.
  • Tests are sufficient.

[jmercouris]

Thanks Andre. I shall have a look!

[jmercouris]

I would have thought it would be enough to swap rqsa for qsa! In any case, it looks good. Thank you.

[jmercouris]

P.S. this is WAY faster.

[aadcg]

I would have thought it would be enough to swap rqsa for qsa! In any case, it looks good. Thank you.

You're missing rqs-nyxt-id. A closer look at b43ec712d450e64605e1a4a167ea6c992a3a036e (restricted to hint-mode) would clarify all that needs to be reverted in the context of this PR.