Withdraw an invitation

Deletes one open invitation on the trip, so its link stops working and it leaves the collaborator list and the cap count. id is the row id of a pending entry on that list. The owner or an admin of the trip may do this; an editor or viewer gets a 403. A 404 when the caller cannot see the trip, the id is not on this trip, or the invitation was already accepted or has expired (remove an accepted person with the v1 collaborator delete instead).