agendaviews: Display recurrence w/o main incidence
Problem: Currently the agenda view hides recurrences which no longer have a main incidence attached to it. Roundcube just displays these recurrences, which seems to preferable compared to simply hiding an event. This patch forces the agenda view to still display a "lost" recurrence. Note about the patch: Removed the `mViewCalendar->isValid(incidence)` check in the if-condition since this is already checked in the first line of the function and simplifies code.