Eliminate old VSA backend

After #271538 (closed) is merged and verified on production, it's time to remove the old VSA code! 🎉

Idea:

We could try deleting the lib/gitlab/cycle_analytics/ folder and the associated spec files. After this, we could run the pipeline and clean up/delete all failing test files.

Note: Group level and project level VSA should work after deleting the files.