REmove unused usings.

src/EditorFeatures/CSharpTest/Structure/InitializerExpressionStructureTests.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;
-using System.Collections.Generic;
-using System.Linq;
-using System.Text;
 using System.Threading.Tasks;
 using Microsoft.CodeAnalysis.CSharp.Structure;
 using Microsoft.CodeAnalysis.CSharp.Syntax;

src/Features/CSharp/Portable/Structure/Providers/InitializerExpressionStructureProvider.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;
 using System.Threading;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
 using Microsoft.CodeAnalysis.Structure;