Remove unnecessary usings

Author: irvinesunday

src/Microsoft.OpenApi/Extensions/StringExtensions.cs

@@ -1,8 +1,7 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
-// Licensed under the MIT license. 
+// Licensed under the MIT license.
 
 using System;
-using System.Linq;
 using System.Reflection;
 using Microsoft.OpenApi.Attributes;