Remove registrant portal from the registry project

[vohmar]

We have not used the old registrant portal that is part of the registry project for a long time now, so it's safe to remove the code from the registry project by now.

New registrant portal is in separate project and is communicating with registry via registrant API
https://github.com/internetee/registrant_center
https://github.com/internetee/registry/tree/master/doc/registrant-api/v1