Don't strip .NETFramework tfms

[ViktorHofer]

With dotnet/runtime#58558, .NETFramework tfms which include a RID (i.e. net461-windows) aren't used and supported in dotnet/runtime anymore. Removing the support from the TargetFramework.Sdk as .NETFramework tfms with a RID aren't supported by NuGet anyways and throw.

To double check:

• The right tests are in and and the right validation has happened. Guidance: https://github.com/dotnet/core-eng/tree/master/Documentation/Validation

[ericstj]

LGTM, please wait to merge this until your change is in on dotnet/runtime side. I'm also assuming you tested this in dotnet/runtime to ensure there are no stragglers using this.