[MX-2] Deletes unused code, cleans up deprecation warnings, removes old Echo flags #3017
This pull request is housecleaning for Eigen. Sorry for the huge PR, but since it's mostly deletions, it didn't make sense to me to split it up.
Overall, the number of native unit tests dropped from 911 to 801. With far less code, I expect to see significantly improved compile times locally for developers, as well as on CI. Users will also see a smaller app download size. A win-win-win
Here's a high-level summary: