Remove deprecated `fixed/highlights` container

[cemms1]

What does this change?

As a follow-up from https://github.com/guardian/frontend/pull/27445, this PR removes the fixed/highlights container now that usages have been migrated to the scrollable/highlights type

Screenshots

Checklist

• [ ] Tested locally, and on CODE if necessary
• [ ] Will not break dotcom-rendering
• [ ] Will not break our database – if updating CAPI, updated and committed the database files
• [ ] Meets our accessibility standards
  • [ ] Tested with screen reader
  • [ ] Navigable with keyboard
  • [ ] Colour contrast passed