Juju must allow ssh keys removal

[Juan M. Tirado]

For design purposes, Juju does not allow to delete all the ssh keys of a model. For example:

juju_29 ssh-keys
Keys used in model: admin/development
ba:fd:cc:51:d3:6b:18:a6:4a:34:19:81:a6:d8:95:82 (cccc@somewhere)

With only one key will show this message when trying to remove the key:

juju_29 remove-ssh-key cccc@somewhere
ERROR cannot delete all keys

This limitation makes no sense for deployments where no SSH access will be done and it directly impacts the UX in other tools from the jujuverse such as the terraform provider.