Remove one of the current user's saved cards

Deletes the card at the gateway (token delete + card-on-file consent revoke) and retires it in the vault. When the primary card is removed and other cards remain, the freshest remaining one becomes primary and live subscriptions are repointed onto it. The only card of a subscription that still renews cannot be removed (400 last_card_on_active_subscription) — the client cancels the autorenew first and retries.