REmove unused imports.

src/EditorFeatures/Test/CodeGeneration/CodeGenerationTests.Shared.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;
 using Microsoft.CodeAnalysis.CodeGeneration;
-using Microsoft.CodeAnalysis.Text;
 using Roslyn.Test.Utilities;
 using Xunit;