Remove unused DSCM plugin files and related components

- Deleted the `datastore.py`, `dscm.py`, `path_resolver.py`, and `requirements.txt` files as they are no longer needed.
- Removed the global store instance and device-specific store logic from `routes.py`.
- Commented out and removed unused route handlers for device configuration in `routes.py`.
- Updated test messages in `dscm_messages.py` to maintain consistency.
- Added backward compatibility in L3VPN handlers