nurse: if the is no repository, unistall all the plugin

It possible fall under a state (if you mess up with a buggy version of coffee) that you have the plugin installed but you have no repository.

So Coffee should remove all the plugins installed when there is no repository.

Start by tracing the nurse command's handling of installed plugins when no repository is configured. Inspect the existing uninstall behavior and add coverage for this state; done means all installed plugins are removed when no repository exists.

Written by the indexing model from the issue text.