onCalendarEventDeleted emits an event identity when incremental synchronization
reports it as cancelled or deleted.
Example
Options
calendarId defaults to "primary" and must be a static calendar ID.
account is an optional static Google account binding.
Trigger data
Returns:event is the last normalized representation when the event existed in the
synchronization baseline; it can be absent. Use top-level eventId and
calendarId as the reliable deletion identity.