[jcalendar] remove jcalendar usage from participant_register_date on event search

[jcalendar] remove jcalendar usage from participant_register_date on event search

This removes one more place where legacy jcalendar is used. There is actually still one left on the form (event date)
which I left out for now as it involves thinking about the fact event is a different entity & might warrant different
representation in the arrays