Remove unused using directive from ValidateConfigTests.cs

Co-authored-by: Aniruddh25 <3513779+Aniruddh25@users.noreply.github.com>

Author: Copilot

src/Cli.Tests/ValidateConfigTests.cs

@@ -1,7 +1,6 @@
 // Copyright (c) Microsoft Corporation.
 // Licensed under the MIT License.
 
-using Azure.DataApiBuilder.Config.ObjectModel;
 using Azure.DataApiBuilder.Core.Configurations;
 using Azure.DataApiBuilder.Core.Models;
 using Serilog;