Remove secondary read-only banner on archived diffs

What does this MR do and why?

Remove secondary read-only banner on diff view when it belongs to an archived group or project.

References

• Closes #570719 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Go to /rails/features and enable the archive_group feature flag.
2. Find or create a merge request and go the Changes tab.
3. Add comments on the diffs.
4. Archive the project/group in Settings > General > Advanced > Archive
5. Go back to the Merge Request > Changes page.

MR acceptance checklist

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