Delete BBM backfill_duo_core_for_existing_subscription

What does this MR do and why?

• This MR deletes the BBM backfill_duo_core_for_existing_subscription which was introduced in !188884 (merged).
• The BBM was finalized in the milestone 18.4 via !202538 (merged).

References

• #573490 (closed): Finalize and remove BackfillDuoCoreForExistingSubscription background migration.
• #539075 (moved): Parent issue describing that one of the tests were quarantined.
• !202538 (merged): MR where BBM backfill_duo_core_for_existing_subscription was finalized (milestone 18.4).

MR acceptance checklist

I have evaluated this MR against the MR acceptance checklist.