This diff removes the rudimental `ping` message from the protobuf.
The "pinging" message mechanism was changed to the [[ https://linear.app/comm/issue/ENG-1842/consider-using-of-grpc-keep-alive-instead-of-ping-messages | gRPC keep-alive mechanism ]] and the server implementation was changed in D5713. The empty "ping" message should be removed from the protobuf API as it's not used anymore.
Linear task: ENG-