mkNugetSource: Remove meta.licenses attribute
- It's useless. The correct attribute name would be `license` and not
`licenses`. Meaning setting this never did anything useful.
- It used IFD in its implementation, meaning to know what the licenses
of a nuget source are, you first had to build that nuget source. This
defeats the point of having license checks in the first place.
- IFD is not allowed by the nixpkgs CI and build farm anyway.