Performing a GET returns something similar to this.

  <service>
    <id>123456789</id>
     -
     -
     -
    <support_email>blah@3scale.com</support_email>
    <tech_support_email/>
    <admin_support_email/>
     -
     -
     -
  </service>

There is nowhere to update the values of <tech_support_email/> and <admin_support_email/> in the front end. There is evidence to suggest that they were removed from the portal a while ago. See https://github.com/3scale/porta/blob/da5d489/lib/developer_portal/lib/liquid/drops/service.rb#L184-L189

Can we confirm that these fields are no longer used and, if so, can we get them removed from the output?

This is not about deleting the fields from the database. Don't do that. Instead, open an issue to remove those fields and set the status to Next please.