Remove retention policy assignment

BoxRetentionPolicyAssignment assignment = new BoxRetentionPolicyAssignment(api, id);
assignment.delete();

client.retentionPolicies.deleteAssignment('12345')
	.then(() => {
		// deletion succeeded — no value returned
	});

client.retentionPolicy.deleteAssignment(assignmentId: "123456")  { result in
    guard case let .success = result else {
        print("Error deleting retention policy assignment")
        return
    }
    
    print("Retention policy assignment was deleted")
}