Hi!
This is a duplicate issue of How to create buttons in email alerting templates? - #3 by pepecano
You cannot remove the “Grouped by” via notification templates, only the the text inside the HTML email table.
You can edit the HTML email template (ng_alert_notification.html) itself.
However, this is only possible in Grafana OSS and is NOT recommended.
This option is not for public usage, and it might change in the future. See Email template personalization - #9 by gillesdemey1