Remove the rapid_diffs_on_mr_show feature flag

What does this MR do and why?

Removes the rapid_diffs_on_mr_show feature flag. It has been enabled by default and on GitLab.com since 19.0, so the merge request page always has Rapid Diffs available.

Behaviour does not change. The Rapid Diffs toggle on the Changes tab stays, and users can still switch to legacy diffs. That choice is still made by the rapid_diffs_enabled cookie, the rapid_diffs and rapid_diffs_disabled URL params, and the rapid_diffs_default_on_mr_show flag. Removing that toggle is out of scope here.

The specs that only covered the flag-off path are removed. The docs get a history entry for the removal. Rapid Diffs stays in Beta, and its [!flag] note on the Changes page stays because rapid_diffs_default_on_mr_show still controls the default.

References

• Rollout issue: #539581
• Introduced in: !187955 (merged)

Screenshots or screen recordings

Not applicable. Nothing changes visibly, because the flag is already enabled everywhere.

How to set up and validate locally

1. Open the Changes tab of any merge request.
2. Confirm the Rapid Diffs toggle is shown, and that Switch to classic loading loads legacy diffs.
3. Select Try Rapid Diffs and confirm Rapid Diffs loads again.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.