Cleanup ArchiveAuthenticationEvents BBM

What does this MR do and why?

Cleanup the ArchiveAuthenticationEvents Batched Background Migration.

• Migration was enqueued in 18.5: !202447 (merged)
  • The migration only targeted gitlab.com. This migration did not target self-managed instances.
• Finalized in 18.6: !210795 (merged)
  • Can verify it is finalized with /chatops run batched_background_migrations status 2001061 --database main
• Since we only targeted .com, there is no need to wait until next required stop to delete the migration, per this conversation

I'm following the docs and removing the BBM code and specs. I noticed in these prior MRs that the enqueue and finalization migrations were deleted as well, so I'm deleting them here too:

• !207795 (diffs)
• !162163 (diffs)
• !200192 (diffs)

References

• Issue: #562154
• https://docs.gitlab.com/development/database/batched_background_migrations/#deleting-batched-background-migration-code
• BBM introduced and enqueued in: !202447 (merged)
• BBM finalized in: !210795 (merged)

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Related to #562154