Remove finalize_after key from BBM dictionary

What does this MR do and why?

finalize_after was initially introduce to auto-finalize the batched background migrations after this date.

But Keeps::OverdueFinalizeBackgroundMigration handles creating finalization MR by directly checking the status in the db clones. With this we can safely remove this key from the BBM dictionary.

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Related to #483064 (closed)