Multiple Participant Registration : Figure out a better way to handle deletion of primary participant

This is a bit of an edge case, but... with the 2.1 implementation we create a single contribution and participant_payment record linked to the "primary" participant when they register for additional folks. Then, if the event registration for the primary participant is deleted - we delete the related contribution / participant_payment records. This leaves the additional participant records as "apparently" NOT PAID for (i.e. no record of payment) - which is not ideal.