[Feature flag clean up] duo_seat_assignment_email_for_sm

What does this MR do and why?

duo_seat_assignment_email_for_sm was defaulted to ON in %17.4, this MR removes the flag.

Related to #481830 (closed)

References

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MR acceptance checklist

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How to set up and validate locally

Same as in !170507 (merged), you will need to simulate a licensed self-managed instance with duo subscription.