Remove obsolete Registry.DynData field from System.Private.CoreLib (dotnet/coreclr...

Remove obsolete Registry.DynData field from System.Private.CoreLib (dotnet/coreclr#5724)

Registry is internal in System.Private.CoreLib due to being pulled-in
by the dependency closure. The obsolete DynData field is only valid
on Win9X, so there's no need to allocate it and keep it around in
CoreCLR. Remove it from System.Private.CoreLib with an ifdef, along
with other references to the HKEY_DYN_DATA hive.

Commit migrated from https://github.com/dotnet/coreclr/commit/0af0ffb17e428a3570e1ae55bfcceb78bdc8eb93