ejabberd config: remove iqdisc option

For me, ejabberd doesn't start since a few days and complains about the unknown option iqdisc.

Unfortunately, plinth sets this explicitly, see https://salsa.debian.org/freedombox-team/plinth/blob/master/actions/ejabberd#L251, but it was recently removed, see upstream blog post https://blog.process-one.net/ejabberd-18-03/. Removing the line in my config file solves the issue.

(It also unmasked another one, I also had to remove the line mod_irc, but plinth doesn't set this, so this is a wider upgrade issue.)

To solve this issue, we should at least remove it from plinth's config. It would be nice if there was a way to inform users, or even upgrade their faulty config files on the next plinth upgrade, if possible? @freedombox-team , any ideas whether we can pull this off?