Delete .NET Native leftovers from type loader (part 2 of X) (#79073)

Delete .NET Native leftovers from type loader (part 2 of X) (#79073)

Main theme: things that deal with individual fields.

I was today years old when I learned we can't just always copy the GCDesc from the template type and sometimes need to construct it from a bitfield. The reason is multidimensional arrays, whose template is always `object[,]` and that GCDesc is only valid for reference-type elements and wrong for everything else.