[FIX] event: prevent scheduled communication for inactive events

Prior this commit:
The mail scheduler of the event notifies the attendees even when the event is
no longer active.

After this commit:
Inactive events no longer notify attendees.

Task-3381876

closes odoo/odoo#136225

X-original-commit: 10c4091f
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>