Use only folded events to hide technical events

We currently have some code in Notifications that checks the type of the events to hide them when the events are grouped in composite events.
This behaviour is right now hardcoded in Notifications, and it's causing bugs (e.g. XWIKI-21059) and it's also lead to some hackish code.
So we should drop that and instead only rely on folded events to ignore events for notifications.

See also: https://forum.xwiki.org/t/remove-hacks-to-hide-technical-events-notifications-when-grouping-them/12718