As such uninstall of the ioserver module removes associated crons. Not sure, how you have removed /var/www/html/admin/modules/iotserver/ module/directory but now as this path does not exists so you have to manually remove the cron. by editing the cron jobs i.e. crontab -u asterisk -e.
I am having this same issue. I have removed it from the system and from the cron but still keep getting these errors. What can I do to stop getting these emails on a module that is not even installed?
If you’ve removed it from crontab, and forced cron to re-read it, the cron jobs won’t try to run, so it will be impossible to receive the email. I assume you either didn’t write the correct file back, or you did so in a way that didn’t signal cron to re-read it.