Remove `scrollable/highlights` container layout definition for frontend

[cemms1]

What is the value of this and can you measure success?

When a front falls back to frontend instead of DCR, we're getting some wacky layouts for scrollable/highlights. Ideally we wouldn't render this container at all but as a first step we can let it fall back to displaying as a fixed/slow/small-iv container as with other containers without definitions.

What does this change?

Removes the layout definition for the scrollable/highlights container and lets it fall back to the default value instead (fixed-4)

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