remove unnecessary using directive

Author: WhatzGames

src/Http/WebUtilities/src/ReasonPhrases.cs

@@ -1,8 +1,6 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-using System.Collections.Frozen;
-
 namespace Microsoft.AspNetCore.WebUtilities;
 
 /// <summary>