[FIX] google_calendar: not sending emails to existing event attendees

Before this commit: syncing with google led to sending emails to
attendees of existing future events on Odoo.

Steps to reproduce the first issue:

- Install 'google_calendar' module
- Integrate with Google Calendar in setting
- Add an event to the Odoo calendar for future date
- Add one external attendee to the event
- Sync with Google

Invitation emails would be sent to the attendees of the events.
Solution

It's possible to not send emails to the attendees in api calls. So
the solution is to not send emails to the attendees for the syncing
time.
opw-2819046

Part-of: odoo/odoo#97012