Remove PSM from NuGet.config

[Youssef1313]

dotnet/sdk#52863 broke us. Darc in official pipeline is installed with --add-source but this doesn't work when PSM is used.

SDK PR breaking us dotnet/sdk#52863

See also dotnet/arcade#16628

dotnet/arcade#16642 might fix the failure differently - but I think this might still be good cleanup anyways :)

[Copilot]

Pull request overview

Removes NuGet Package Source Mapping (PSM) and related configuration from the repo-root NuGet.config to avoid failures introduced by dotnet tool install --add-source behavior changes (impacting Darc installation in official pipelines).

Changes:

• Removed <packageSourceMapping> so tool installs/restores can honor --add-source.
• Removed <activePackageSource> and empty dependency-flow-managed comment blocks from NuGet.config.

[Youssef1313]

Looks like we cannot delete PSM:

src\Package\MSTest.Sdk\MSTest.Sdk.csproj(0,0): error NU1507: (NETCORE_ENGINEERING_TELEMETRY=Restore) There are 7 package sources defined in your configuration. When using central package management, please map your package sources with package source mapping (https://aka.ms/nuget-package-source-mapping) or specify a single package source. The following sources are defined: test-tools, dotnet-public, dotnet-tools, dotnet-eng, dotnet9, dotnet10, dotnet11