seal up unsealed classes, clean up using statements

src/Untappd.Net.UnitTests/Responses/Actions/TestCheckInAction.cs

@@ -1,8 +1,4 @@
 using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Text;
-using System.Threading.Tasks;
 using NUnit.Framework;
 using Untappd.Net.Responses.Actions;