Merge pull request #13835 from dpoeschl/FixUsingsForCyrus

Fix usings in CSharpTypeStyleDiagnosticAnalyzerBase

src/Features/CSharp/Portable/Diagnostics/Analyzers/CSharpTypeStyleDiagnosticAnalyzerBase.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
-using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Diagnostics;
 using System.Linq;
@@ -12,7 +11,6 @@
 using Microsoft.CodeAnalysis.Text;
 using Microsoft.CodeAnalysis.CSharp.CodeStyle;
 using Microsoft.CodeAnalysis.CodeStyle;
-using System;
 
 namespace Microsoft.CodeAnalysis.CSharp.Diagnostics.TypeStyle
 {