Cleanup ArchiveRevokedAccessGrants background migration

What does this MR do and why?

Cleanup ArchiveRevokedAccessGrants background migration

Remove code for the finalized ArchiveRevokedAccessGrants batched background migration following the standard cleanup process.

• Remove background migration class and spec
• Remove queue migration spec
• Remove migration documentation

The migration was finalized in 18.6 and is now safe to clean up.

Related to: #562376

Additional notes

• Migration was enqueued in 18.5: !207837 (merged)
  • The migration only targeted gitlab.com. This migration did not target self-managed instances.
• Finalized in 18.6: !211207 (merged)
  • Can verify it is finalized with /chatops run batched_background_migrations status 2001064 --database main
• Since we only targeted .com, there is no need to wait until next required stop to delete the migration, per this conversation

References

• Issue: #562376
• https://docs.gitlab.com/development/database/batched_background_migrations/#deleting-batched-background-migration-code
• BBM introduced and enqueued in: !207837 (merged)
• BBM finalized in: !211207 (merged)

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