_lacme-www shouldn't own any file or directories, but there might be
files on disk owned by _lacme-client when 'challenge-directory' is used.

See https://wiki.debian.org/AccountHandlingInMaintainerScripts#Reasons_for_not_deleting_accounts .

  * The internal webserver now runs as a dedicated system user _lacme-www
    (and group nogroup) instead of www-data:www-data.  This is configurable
    in the [webserver] section of the lacme(8) configuration file.
  * The internal ACME client now runs as a dedicated system user _lacme-client
    (and group nogroup) instead of nobody:nogroup.  This is configurable in
    the [client] section of the lacme(8) configuration file.
  * The _lacme-www and _lacme-client system users are created automatically by
    lacme.postinst (hence a new Depends: adduser), and deleted on purge.  (So
    make sure not to chown any file to these internal users.)