React to https://github.com/dotnet/runtime/pull/101461 (System.IO.Pipelines)

https://github.com/dotnet/runtime/pull/101461 adds System.IO.Pipelines to dotnet/runtime's Shared Framework. We should remove it from ours, update our SharedFx/Targeting Pack tests, and remove all explicit references to System.IO.Pipelines since they'll now be unnecessary