[services] Tunnelbroker - Remove checkpoints from the database

Summary:
In past research, we decided to switch from the time-based message checkpoints to the unique message Ids because of the complexity of the time synchronization between the service instances. The checkpoints-related fields are still in the database model and should be removed.

The full context is in ENG-1158.

Linear task: ENG-2357

Test Plan: Service is successfully built.

Reviewers: jon, varun, tomek

Reviewed By: tomek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D5733