Remove dotnet-core feed (#11663)

Allows removal of the nuget scanning opt-out. I can migrate any packages on this feed over to the appropriate location.

NuGet.config

@@ -6,7 +6,6 @@
   <!-- Only specify feed for Arcade SDK (see https://github.com/Microsoft/msbuild/issues/2982) -->
   <packageSources>
     <clear />
-    <add key="dotnet-core" value="https://dotnetfeed.blob.core.windows.net/dotnet-core/index.json" />
     <add key="dotnet-eng" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-eng/nuget/v3/index.json" />
     <add key="dotnet-tools" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-tools/nuget/v3/index.json" />
     <add key="dotnet5" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet5/nuget/v3/index.json" />