Audit VRT scenarios and prune low value cases

## Feature Description

When we originally migrated to Storybook v6 and Component Story Format stories, one of the goals was to make it easier to co-locate VRT scenario definitions with their respective stories. However, since then, one side-effect has been our set of VRT scenarios growing substantially. Today, the VRT step alone takes over 20 minutes to complete in GitHub actions which is our longest-running job.

Part of the issue is that Backstop did not previously support scenario-level viewport configuration, so all scenarios ended up 3 images (one for each defined viewport).

This issue is to review the scenarios we currently have defined and identify any which aren't adding much value and can be removed.

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## Acceptance criteria

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## Implementation Brief

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### Test Coverage

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## QA Brief

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## Changelog entry

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