[FIX] mass_mailing: mailing.mailing does not use template_id

Steps to reproduce:
- install crm and mass_mailing
- go to crm > configuration > settings > activate leads
- go to crm > leads > select at least 2 leads > action > send mail
- set a subject > set a mass mailing name > send

Previous behavior:
you get a traceback
ValueError: Invalid field 'template_id' on model 'mailing.mailing'

Current behavior:
the mass mail is sent

opw-2212605

closes odoo/odoo#48088

X-original-commit: c87db90d5dbab4d02a9378a1fc905c9b2935cbe2
Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>
Signed-off-by: mightyjol <jhk-odoo@users.noreply.github.com>

addons/mass_mailing/wizard/mail_compose_message.py

@@ -30,7 +30,7 @@ class MailComposeMessage(models.TransientModel):
                 mass_mailing = self.env['mailing.mailing'].create({
                         'campaign_id': self.campaign_id.id,
                         'name': self.mass_mailing_name,
-                        'template_id': self.template_id.id,
+                        'subject': self.subject,
                         'state': 'done',
                         'reply_to_mode': reply_to_mode,
                         'reply_to': reply_to,