remove unnccessary usings

Author: Jasmine

src/GitHub.Exports/Services/IMenuHandler.cs

@@ -1,5 +1,4 @@
 using System;
-using System.Threading.Tasks;
 
 namespace GitHub.VisualStudio
 {

src/GitHub.VisualStudio/GitHubPackage.cs

@@ -4,7 +4,6 @@
 using System.IO;
 using System.Reflection;
 using System.Runtime.InteropServices;
-using SysTask = System.Threading.Tasks;
 using GitHub.Extensions;
 using GitHub.Models;
 using GitHub.Services;