Revert "chore: add local NuGet source via NOSCORE_LOCAL_PACKAGES env var"

NuGet emits NU1301 (hard error, not warning) when the env var is unset,
so fresh clones and CI runners without NOSCORE_LOCAL_PACKAGES fail restore.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: erwan-joly

NuGet.config

@@ -1,7 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <configuration>
     <packageSources>
-        <add key="local" value="%NOSCORE_LOCAL_PACKAGES%" />
         <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
     </packageSources>
 </configuration>