Make RuntimeType sealed and deleted support for introspection only loads (dotnet/coreclr#19427)
- Sealed Runtime makes `is RuntimeType` and similar checks faster. These checks are fairly common in reflection.
- Delete support for introspection only loads from the runtime. We do not plan to use in .NET Core. The support for introspection loads inherited from RuntimeType and thus it is incompatible with sealed RuntimeType.
Commit migrated from https://github.com/dotnet/coreclr/commit/
af4ec7c89d0192ad14392da04e8c097da8ec9e48