remove unnecessary nullable enable

Author: zivkan

src/NuGet.Clients/NuGet.PackageManagement.VisualStudio/Projects/LegacyProjectAdapter.cs

@@ -1,8 +1,6 @@
 // Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
-#nullable enable
-
 using System;
 using System.Collections;
 using System.Collections.Generic;