Remove nullability attributes generator

[Sergio0694]

This PR removes the nullability attributes generator, which is a change long overdue. This generator was just there to keep the code simpler when generating transitive members (eg. from [INotifyPropertyChanged]), but in hindsight pushing the extra complexity onto final users was not the right choice, as it resulted in somewhat unintuitive behavior. This is because users would find themselves with a couple of random polyfills for [NotNull] and [NotNullIfNotNull], which would not be expected and not really documented either, and also it could potentially conflict with other generators doing that explicitly.

This PR removes the generator entirely, and just updates the code to strip those attributes during generation if not available. That is, if those attributes are not available, the code will simply be generated without using those attributes at all.

PR Checklist

• Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
• Based off latest main branch of toolkit
• PR doesn't include merge commits (always rebase on top of our main, if needed)
• Tested code with current supported SDKs
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Header has been added to all new source files (run build/UpdateHeaders.bat)
• Contains NO breaking changes
• Every new API (including internal ones) has full XML docs
• Code follows all style conventions