Redundant plone.app.event

[folix-01]

The package plone.app.event seems to be used nowhere

[davisagli]

It is loaded as a dependency of plone.app.contenttypes: https://github.com/plone/plone.app.contenttypes/blob/master/plone/app/contenttypes/configure.zcml#L38

Probably its functionality can be carefully moved into plone.app.contenttypes, but we should try to minimize any need for upgrade steps.

[mauritsvanrees]

See #664, but not ready, and a bit tricky to do right, at least in Plone 6.0.