Remove unused using

Author: Youssef1313

src/Utilities/Compiler/Extensions/IEnumerableOfIMethodSymbolExtensions.cs

@@ -1,7 +1,6 @@
 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the MIT license.  See License.txt in the project root for license information.
 
 using System.Collections.Generic;
-using System.Collections.Immutable;
 using System.Linq;
 using Microsoft.CodeAnalysis;