Unsubscribe a user from a list
Example
Method: DELETE
URL: https://api.courier.com/lists/abcdefgh12345678/subscriptions/0460766e-8463-4905-ae98-b72c7aef41d6
Method: DELETE
URL: https://api.courier.com/lists/abcdefgh12345678/subscriptions/0460766e-8463-4905-ae98-b72c7aef41d6