Sorry, the template needs to be removed manually.
api-cli run module/metrics1/configure-module --data '{"prometheus_path": "prometheus", "grafana_path": "grafana", "mail_to": ["alert@nethserver.org"], "mail_from": "no-reply@nethserver.org", "mail_template": ""}'
To apply the changes restart the service:
runagent -m metrics1 systemctl --user restart alertmanager