Fixes #3443 - clean up unused/unimplemented code from existing analyzer project

[dodexahedron]

These are the changes for https://github.com/dodexahedron/Terminal.Gui/issues/41.

Just cleanup before continuing work on Roslyn stuff.

All work on #3443 starts from the end of this.

Can be merged now or can just wait til the entire body of work is done and merge that all at once.

There are no functional changes contained in this PR except that some extra design-time-only code for one of the removed classes will not be generated in the Terminal.Gui project any more.

Fixes

• Cleanup of code from #3420 before continuing with #3443

Proposed Changes/Todos

• [x] Removed no-longer-planned generator that the new one will make redundant and do better anyway.
• [x] Cleaned up a bit of code in the EnumExtensionMethodsGenerator for features I decided not to add to it.
• [x] Took a few R# suggestions and silenced a couple of inspections for things that were intentional.

Pull Request checklist:

• [x] I've named my PR in the form of "Fixes #issue. Terse description."
• [x] My code follows the style guidelines of Terminal.Gui - if you use Visual Studio, hit CTRL-K-D to automatically reformat your files before committing.
• [x] My code follows the Terminal.Gui library design guidelines
• [x] I ran dotnet test before commit
• [x] I have made corresponding changes to the API documentation (using /// style comments)
• [x] My changes generate no new warnings
• [x] I have checked my code and corrected any poor grammar or misspellings
• [x] I conducted basic QA to assure all features are working

[dodexahedron]

Note this is branched from after #3438, so it'll show those changes in the changes above until that's merged.