Remove `--cert` option from `cadc-get-cert`

[andamian]

This reveals a weakness in our system. Better to remove it from here now.

[chitrangpatel]

Are there plans to replace this system with token or ssh based authentication? That would make life easy for building automated systems to do file uploads.

[andamian]

Yes. Support for tokens is being deployed to our services. It will be added to the Python clients after that.