[FIX] event: fix email sent on duplicates

- Create an event
- Add a mail reminder for it
- Run the mail scheduler manually
- Duplicate the event

The mail reminder is sent on the duplicate too.
This commit remove the field `mail_sent` from duplicating.

OPW-2117343

closes odoo/odoo#39742

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>