Clean-up unused feeds

[ViktorHofer]

It would be good to remove unused nuget feeds when removing unused packages - this should probably happen as part of the agentic workflow / skill. The current set of feeds grew quite large before #1704

[mthalman]

@akoeplinger - Was #1704 intending to fix this completely or is there more work?

[akoeplinger]

@mthalman that was a one-off cleanup. I think Viktor intended for this to be something that happens periodically.

[ViktorHofer]

There's a skill / agentic workflow in this repo that removes unused packages. It would be good to also remove unused packages as part of that.