Remove TypedReference.MakeTypedReference readonly restriction (dotnet/coreclr#21193)

Remove TypedReference.MakeTypedReference readonly restriction (dotnet/coreclr#21193)

TypedReference.MakeTypedReference validates that the fields aren't readonly, which limits our ability to make fields readonly in core while maintaining BinaryFormatter compatibility.  This restriction isn't useful for a variety of reasons, though: reflection allows readonly fields to be set, but more importantly, TypedReference.SetTypedReference has always thrown NotSupportedException, so it's not even actually usable.

This change just removes the readonly check from MakeTypedReference.

Commit migrated from https://github.com/dotnet/coreclr/commit/a6729dab35fde5d50fb12b06aeb77d3e1f3be872