duplicate email fields in Cart checkout

Reported by kurund: the ParticipantsAndPrices page in the cart workflow shows two email fields for each participant profile.

This probably has to do with changes in CRM-9587, and the solution is to remove hardcoded "email" fields in the Cart participant profile. We'll have to identify contacts by another unique key, hopefully contact_id is now consistent when a temporary participant in the "Deleted" status is converted into a live participant.