remove extraneous imports.

src/Compilers/CSharp/Portable/Symbols/Metadata/PE/PEAssemblySymbol.cs

@@ -5,7 +5,6 @@
 using Roslyn.Utilities;
 using System.Diagnostics;
 using System.Linq;
-using System;
 
 namespace Microsoft.CodeAnalysis.CSharp.Symbols.Metadata.PE
 {

src/Compilers/CSharp/Portable/Symbols/Retargeting/RetargetingModuleSymbol.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.Concurrent;
 using System.Collections.Generic;
 using System.Collections.Immutable;

src/Compilers/CSharp/Portable/Symbols/Source/SourceModuleSymbol.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.Collections.Immutable;
 using System.Diagnostics;
@@ -8,7 +7,6 @@
 using System.Reflection.PortableExecutable;
 using System.Runtime.InteropServices;
 using System.Threading;
-using Microsoft.CodeAnalysis.CSharp.Emit;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
 using Roslyn.Utilities;
 

src/Workspaces/Core/Portable/FindSymbols/SymbolTree/SymbolTreeInfo.cs

@@ -2,13 +2,11 @@
 
 using System;
 using System.Collections.Generic;
-using System.IO;
 using System.Linq;
 using System.Runtime.CompilerServices;
 using System.Threading;
 using System.Threading.Tasks;
 using Roslyn.Utilities;
-using static Roslyn.Utilities.PortableShim;
 
 namespace Microsoft.CodeAnalysis.FindSymbols
 {