ApartmentState STA/MTA is only used to interop with COM and not supported on non-Windows. It also seems that dotnet can marshal between STA and MTA just fine so allowing the user to set it is just a perf optimization.

I propose we simplify our code by removing support. The code base today already has ApartmentState commented out as we run MTA by default (as that is default for dotnet) so this is simply removing the #if !CORECLR conditional code.