Delete dead code (dotnet/coreclr#18783)

Delete dead code (dotnet/coreclr#18783)

- Leftover from unobserved exception desktop quirk
- Unnecessary calls to AppDomain.IsFinalizingForUnload(). They were actually never required even with AppDomain support because of Environment.HasShutdownStarted returns true during AppDomain unload.

Commit migrated from https://github.com/dotnet/coreclr/commit/cd1232d47cc028ebf6d22621ce63903a7e5c0e94