Where possible, remove code copied from .NET Framework

When porting to .NET Core, we copied (and in some cases modified) code from the .NET Framework for functionality that wasn't available in .NET Core. The largest parts of this code were for `XmlTextWriter`, `GetSpecialFolders`, and method binding/invocation logic mostly from `RuntimeType` and `DefaultBinder`. Most of this code is in the `src\Shared\Compat` folder of the repo.

We should discuss with the .NET Framework team whether there are alternatives that we should be using instead, or if this functionality should be added to .NET Core.