Remove unused usings.

src/EditorFeatures/Test/Completion/AbstractCompletionProviderTests.cs

@@ -6,14 +6,12 @@
 using System.Security;
 using System.Threading;
 using System.Threading.Tasks;
-using System.Windows.Threading;
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.Completion;
 using Microsoft.CodeAnalysis.Editor.Implementation.IntelliSense.Completion;
 using Microsoft.CodeAnalysis.Editor.UnitTests.Workspaces;
 using Microsoft.CodeAnalysis.LanguageServices;
 using Microsoft.CodeAnalysis.Options;
-using Microsoft.CodeAnalysis.Shared.Extensions;
 using Microsoft.CodeAnalysis.Text;
 using Microsoft.VisualStudio.Language.Intellisense;
 using Microsoft.VisualStudio.Text;