netcoreapp3.0 support should be removed

[MichaelSimons]

The SBRP generate tooling supports netcoreapp3.0 but the tfm filtering will filter it out. The result is unused code gets created. To fix this the following changes should be made:

1. Add netcoreapp3.0 as an ExcludeTargetFrameworks
2. Remove the TargetingPackVersion handling for netcoreapp3.0
3. Regenerate the reference packages that target netcoreapp3.0