Removing unintentional import statement

Author: Stanley Goldman

src/GitHub.App/ViewModels/GistCreationViewModel.cs

@@ -8,7 +8,6 @@
 using GitHub.Extensions;
 using GitHub.Models;
 using GitHub.Services;
-using Microsoft.VisualStudio.Shell;
 using NLog;
 using NullGuard;
 using Octokit;