C#: Remove some dead code.

Author: michaelnebel

csharp/extractor/Semmle.Extraction.CSharp.DependencyFetching/NugetPackageRestorer.cs

@@ -614,14 +614,6 @@ public void Dispose()
             feedManager.Dispose();
         }
 
-        /// <summary>
-        /// Returns the full path to a temporary directory with the given subfolder name.
-        /// </summary>
-        private static string ComputeTempDirectoryPath(string subfolderName)
-        {
-            return Path.Join(FileUtils.GetTemporaryWorkingDirectory(out _), subfolderName);
-        }
-
         /// <summary>
         /// Computes a unique temporary directory path based on the source directory and the subfolder name.
         /// </summary>