PR Summary

Fixes #15535

AppDomain.CreateAppDomain and AppDomain.Unload were used for binary module analysis originally -- loading the binary module into another AppDomain so the analysis doesn't pollute the current app domain. However, the code path was disabled when moving to .NET Core because there has been only one AppDomain since the beginning in .NET Core.

That code path was for analyzing a DLL module (without a module manifest .psd1 file), and it's really not that useful in real world scenarios because all modules should have a module manifest file, which defines what commands are exposed from the module. Therefore, nothing is really affected even though that code path was disabled since 2016.

This cleanup removed that code path that creates an AppDomain all together, as well as the relevant code for cleaning up the AppDomain.

Also, a simple NullReferenceException is fixed in Parser.ParseInput.

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