Remove unused usings, add one missed from rebase

Author: Jasmine

src/GitHub.VisualStudio/Base/MenuBase.cs

@@ -5,9 +5,8 @@
 using GitHub.Services;
 using System.Threading.Tasks;
 using GitHub.Api;
-using GitHub.Primitives;
 using NullGuard;
-using System.Diagnostics;
+using GitHub.UI;
 
 namespace GitHub.VisualStudio
 {

src/GitHub.VisualStudio/Menus/CopyLink.cs

@@ -5,7 +5,7 @@
 using GitHub.Extensions;
 using GitHub.Services;
 using GitHub.VisualStudio.UI;
-using System.Threading.Tasks;
+using NullGuard;
 using GitHub.Api;
 
 namespace GitHub.VisualStudio.Menus

src/GitHub.VisualStudio/Menus/OpenLink.cs

@@ -3,7 +3,7 @@
 using Microsoft.VisualStudio.Shell;
 using System;
 using System.ComponentModel.Composition;
-using System.Threading.Tasks;
+using NullGuard;
 using GitHub.Api;
 
 namespace GitHub.VisualStudio.Menus