Remove incorrect using statement from ASPNET Owin (#10475)

articles/quickstart/webapp/aspnet-owin/files/account.controller.md

@@ -6,7 +6,6 @@ language: csharp
 ```csharp
 using Microsoft.AspNetCore.Authentication;
 using Microsoft.AspNetCore.Authentication.Cookies;
-using Auth0.AspNetCore.Authentication;
 
 public class AccountController : Controller
 {
@@ -44,4 +43,4 @@ public class AccountController : Controller
     HttpContext.GetOwinContext().Authentication.SignOut("Auth0");
   }
 }
-```
+```