Remove references to PlatformAbstractions.

[eerhardt]

This will allow us to remove the PlatformAbstractions library from dotnet/runtime.

Contributes to dotnet/runtime#3470

I couldn't find any callers of the GetTargetMachineInfo task. I'm not sure I looked everywhere, but since it is in the SharedFramework.Sdk package, it should only be used by runtime, aspnetcore, or windowsdesktop repos, right?

[dagood]

[...] the SharedFramework.Sdk package, it should only be used by runtime, aspnetcore, or windowsdesktop repos, right?

Just runtime and windowsdesktop. aspnetcore, standard, and installer are other places that could potentially use it, but don't.

(Edit: Added more to the quote, but I do see how I'm kind of warping it and my comment was unclear.)

[ViktorHofer]

Runtime shouldn't use that task anymore so I assume just windowsdesktop?

[eerhardt]

Runtime shouldn't use that task anymore so I assume just windowsdesktop?

I believe @dagood was referring to this part:

but since it is in the SharedFramework.Sdk package

What I read is that only runtime and windowsdesktop repos use the SharedFramework.Sdk package.