[FIX] base,calendar: fix calendar reminder

The purpose of this commit is to have reliable calendar event reminder.

Before, the reminders was not send directly and was instead put in mailing
queue, which was leading to some delay. This is due do mail_notify_force_send
context key being force to False when being in cron mode. However here we
do not want to have calendar cron scheduling emails, then mailing cron
sending them as it implies two delays.

In order to try to send reminder mails as soon as possible compared to their
schedule let us send mails directly during the calendar cron.

Task ID-2331999

closes odoo/odoo#57994

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>