Removing user via command-line tool should also delete user data from brokers

[adombeck]

The new command-line tool (WIP in #782) will allow to remove users from the authd database. However, in it's current design, it does not remove the user data stored by the broker in /var/snap/<broker>/current/<issuer>/<user>. That directory contains the hashed local password and the token.json with the tokens and UserInfo. That data should also be deleted when the command-line tool is used to remove a user.

The command-line tool talks to authd and should, in my opinion, not talk to the broker directly. Since authd doesn't know where the brokers store this data, I think the brokers should provide a DeleteUser API method which is called by authd.