我使用Azure移動服務Facebook身份驗證創建了.NET應用程序。我用憑證:Azure移動服務 - Facebook登出
// Login with the identity provider.
user = await App.MobileService
.LoginAsync(provider);
// Create and store the user credentials.
credential = new PasswordCredential(provider,
user.UserId, user.MobileServiceAuthenticationToken);
vault.Add(credential);
如何註銷?