Remove unnecessary using

Author: MaggieKimani1

test/Microsoft.OpenApi.Tests/Writers/OpenApiJsonWriterTests.cs

@@ -8,7 +8,6 @@
 using System.IO;
 using System.Linq;
 using System.Text;
-using System.Text.Json;
 using FluentAssertions;
 using Microsoft.OpenApi.Any;
 using Microsoft.OpenApi.Models;