This can be removed by modifying (commenting the line) the qweb view 

Mail: mail notification layout template 

Settings > Technical > View (you would need enable developer mode to view this menu)

There is no such feature in the Helpdesk Email templates, at least not in Odoo 16 self hosted, as far I know. Total nonsense, because as paying customer the least one would want is a customizable template.

The very few customisable templates are visible in Settings > Technical > Email > Email Templates

There are only 3 templates there though:
Helpdesk: Request Acknowledgment

Helpdesk: Ticket Closed

Helpdesk: Ticket Rating Request

It is extremely frustrating that there is zero documentation on how to create, say, a template for "Follow up" mails, and how to trigger a specific email template upon a specific ticket action. Not to mention that the templates it lets you customize, are factually only placeholders for inner text - the emails are still wrapped in a UGLY looking and branded "container"

I wish Odoo would at least inform us if this is _somehow_ customisable or just plain out impossible.