Delete UserAccount

[jrse]

Analyse possible ways to delete a mail account and test, if the existing doveadm command to delete a mailbox works with dovecot-ceph-plugin.

[jrse]

doveadm mailbox delete command can be used to delete a user mailbox.
Alternatively doveadm rmb mailbox delete command can be used.

In case indirect namespace is used, the 90-plugin configuration rados_check_empty_mailboxes=true should be set to delete the namespace user mapping file.