Feature flag removal: `registry_migration_guard_thresholds`

What does this MR do and why?

Removes the registry_migration_guard_thresholds feature flag. This flag has been enabled on GitLab.com since 2022-05-17. This feature flag was a safety guard for a change to the container registry phase 2 migration, which is only occurring on GitLab.com right now. The entire migration is behind a feature flag, so technically, this update is still behind a feature flag which is why I chose a changelog of other rather than added/changed.

Screenshots or screen recordings

N/A

How to set up and validate locally

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MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #350543