Removed unneeded method.

Author: grokys

src/GitHub.App/SampleData/PullRequestDetailViewModelDesigner.cs

@@ -125,11 +125,6 @@ public PullRequestDetailViewModelDesigner()
 
         public Task InitializeAsync(ILocalRepositoryModel localRepository, IConnection connection, string owner, string repo, int number) => Task.CompletedTask;
 
-        public Task<string> ExtractFile(IPullRequestFileNode file, bool head)
-        {
-            return null;
-        }
-
         public string GetLocalFilePath(IPullRequestFileNode file)
         {
             return null;