Remove unused using

src/Workspaces/Core/Portable/Workspace/Solution/ProjectState.cs

@@ -13,7 +13,6 @@
 using System.Threading.Tasks;
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.Diagnostics;
-using Microsoft.CodeAnalysis.Emit;
 using Microsoft.CodeAnalysis.Host;
 using Microsoft.CodeAnalysis.Serialization;
 using Roslyn.Utilities;