Remove backups from NixOps core

[adisbladis]

Backups is only implemented for the EC2 backend and shouldn't be in NixOps core.

We should implement the appropriate hooks so that plugins can extend the CLI.

[tewfik-ghariani]

It is used for GCP as well ^^

https://github.com/nix-community/nixops-gce/blob/master/nixops_gcp/backends/gce.py#L1048