[FIX] mail: remove unnecessary model decorator

A method that actually runs on recordsets should not have a model decorator.
Even if there is no issue this is may be confusing.

Task-2710804 (Mail: Clean Mail.Thread API)

Part-of: odoo/odoo#82167

addons/mail/models/mail_thread.py

@@ -2350,7 +2350,6 @@ class MailThread(models.AbstractModel):
 
         return True
 
-    @api.model
     def _notify_by_email_prepare_rendering_context(self, message, msg_vals=False, model_description=False,
                                                    force_email_company=False, force_email_lang=False):
         """ Prepare rendering context for notification email.