Remove split_ghost_user_migration_queue_into_human_and_non_human FF

What does this MR do and why?

This split_ghost_user_migration_queue_into_human_and_non_human FF has been enabled on GitLab.com since 2026-06-03T09:59:43Z, see https://gitlab.com/gitlab-com/gl-infra/feature-flag-log/-/work_items/59477.

This MR removes the FF to enable improvements behind this FF on GitLab Self-Managed.

Related to #602410 (closed)

References

• [FF] `split_ghost_user_migration_queue_into_hum... (#601101 - closed)
• [Feature flag] Cleanup `split_ghost_user_migrat... (#602410 - closed)

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