Remove unnecessary using

Author: MaggieKimani1

src/Microsoft.OpenApi/Writers/OpenApiWriterBase.cs

@@ -6,7 +6,6 @@
 using System.IO;
 using System.Linq;
 using System.Text.Json;
-using System.Xml.Linq;
 using Json.Schema;
 using Json.Schema.OpenApi;
 using Microsoft.OpenApi.Any;