Stop creating keep-arounds on merge-request notes

What does this MR do and why?

Merge requests already keep all associated commits. When notes create keep-arounds on merge requests they are always superfluous - created on commit chains that are already kept.

This will reduce the total number of keep-around refs create on .com by roughly 17k refs/day.

References

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• Epic Phase 1: Keep-around ref optimization (&15630)
• Fixes Optimize merge request keep-around refs (#500160 - closed)

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