remove unused using

Author: Morilli

src/BizHawk.Tests/Client.Common/config/SerializationStabilityTests.cs

@@ -1,7 +1,6 @@
 using System;
 using System.Collections.Generic;
 using System.Drawing;
-using System.Globalization;
 using System.Linq;
 using System.Reflection;
 using System.Text.Json;