Remove logic that moves models out of models directory once migration is complete

[JoshLove-msft]

After the migration to the new generator is done, we should remove the logic that was added to leave models out of the models directory, which was added to make cleaner diffs.

[JoshLove-msft]

I think we can just remove this now even though the migration is not done. The GitHub diffs are easy to understand.

[JoshLove-msft]

This is in the NamespaceVisitor