[Digital Clock] Hide agenda if no calendar event plugins are enabled

The agenda doesn't add a lot of usefulness if there aren't any appointments to be shown.
The weekday and date is also shown in the tooltip and I can see that people might prefer
the older more compact look.

BUG: 364455
FIXED-IN: 5.7.1

Differential Revision: https://phabricator.kde.org/D2056