Fix formatting.

Author: westey-m

dotnet/src/Agents/Core/ChatCompletionAgent.cs

@@ -3,7 +3,6 @@
 using System;
 using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
-using System.Linq;
 using System.Runtime.CompilerServices;
 using System.Text;
 using System.Text.Json;