Remove hacspec crypto backend

- it is not needed anymore -- since the addition of the `rustcrypto` backend, which now serves as the default backend for non-embedded environments
- and crucially, it has been [causing CI to fail](https://github.com/openwsn-berkeley/lakers/actions/runs/7831541740/job/21368143161), with a hard to diagnose cause (it was pulled from a stale fork)