Restore single-feed nuget.config (CFS only)

PR #267 had added nuget.org as a NuGet source and a packageSourceMapping
that routed * to nuget.org, plus a hardcoded local feed at C:\Users\Tig.
This caused Terminal.Gui (and other dependencies) to be pulled directly
from nuget.org instead of the PowerShell team's CFS feed, violating the
policy that all packages flow through CFS for upstream-mirrored ingestion.

Revert nuget.config to main's contents: a single PowerShellCore_PublicPackages
source. CFS upstream-ingests from nuget.org on demand, so no source mapping
is needed and Terminal.Gui 2.0.1 (and everything else) restores from CFS.

Verified: project.assets.json + .nupkg.metadata both show Terminal.Gui 2.0.1
sourced from pkgs.dev.azure.com/powershell/PowerShell/_packaging/powershell.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: andyleejordan

nuget.config

@@ -1,18 +1,7 @@
-<?xml version="1.0" encoding="utf-8"?>
+<?xml version="1.0" encoding="utf-8"?>
 <configuration>
   <packageSources>
     <clear />
     <add key="PowerShellCore_PublicPackages" value="https://pkgs.dev.azure.com/powershell/PowerShell/_packaging/powershell/nuget/v3/index.json" />
-    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
-    <add key="local" value="C:\Users\Tig/psrepo" />
   </packageSources>
-  <packageSourceMapping>
-    <packageSource key="nuget.org">
-      <package pattern="*" />
-    </packageSource>
-    <packageSource key="PowerShellCore_PublicPackages">
-      <package pattern="Microsoft.PowerShell.*" />
-      <package pattern="System.Management.Automation" />
-    </packageSource>
-  </packageSourceMapping>
 </configuration>