Remove unnecessary using statement

Author: joseharriaga

tests/Assistants/Assistants.VectorStoresTests.cs

@@ -9,7 +9,6 @@
 using System.Collections.Generic;
 using System.Linq;
 using System.Text.Json;
-using System.Threading;
 using System.Threading.Tasks;
 using static OpenAI.Tests.TestHelpers;