Takes a prospect out of the send-next queue. The prospect stays in the ICP and keeps their normal place in the sending order; copy sent with send_next is kept. If the queue had already handed the email to a mailbox (status SENDING, a window of up to ten minutes), that send is cancelled too and cancelledInFlight is true; the prospect's next regular turn may still send it. The workspace must be a concrete id; all is not accepted here.