[quick fix] Remove default.NuGet.config (#29027)

- always copy the repo's NuGet.config file to Helix agents

eng/helix/content/RunTests/TestRunner.cs

@@ -26,13 +26,6 @@ namespace RunTests
         {
             try
             {
-                // Rename default.NuGet.config to NuGet.config if there is not a custom one from the project
-                // We use a local NuGet.config file to avoid polluting global machine state and avoid relying on global machine state
-                if (!File.Exists("NuGet.config"))
-                {
-                    File.Copy("default.NuGet.config", "NuGet.config");
-                }
-
                 EnvironmentVariables.Add("PATH", Options.Path);
                 EnvironmentVariables.Add("helix", Options.HelixQueue);
 

eng/helix/content/default.NuGet.config

@@ -1,7 +0,0 @@
-<?xml version="1.0" encoding="utf-8"?>
-<configuration>
-  <packageSources>
-    <clear />
-    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
-  </packageSources>
-</configuration>