gitlab: remove unused using statements

Author: mjcheetham

src/shared/GitLab.Tests/GitLabAuthenticationTests.cs

@@ -1,10 +1,6 @@
 using System;
-using System.Collections.Generic;
-using System.IO;
 using System.Threading.Tasks;
 using GitCredentialManager.Tests.Objects;
-using Moq;
-using Moq.Protected;
 using Xunit;
 
 namespace GitLab.Tests

src/shared/GitLab/GitLabAuthentication.cs

@@ -1,7 +1,6 @@
 using System;
 using System.Threading.Tasks;
 using System.Collections.Generic;
-using System.Globalization;
 using System.Net.Http;
 using System.Text;
 using System.Threading;

src/shared/GitLab/GitLabConstants.cs

@@ -1,6 +1,4 @@
 using System;
-using System.Collections.Generic;
-using System.Linq;
 
 namespace GitLab
 {