[ticket/14212] Adding event after users have been removed to a group

The idea is to be able to map with others CMS by exposing most commons user configuration & changes.
Difference between the event before and after group belonging removal is that
users belongs to multiple groups, so that the belonging into a group also
depends on the others groups (and not only the deleted one), which is not
reflected before as the user is not yet deleted

PHPBB-14212