korgac: hide "Dismiss all" button if there's only one reminder.

Summary: It's confusing to have two buttons that do the exact same thing.

Test Plan: Reminder dialog now only shows "Dismiss reminder" when there is only one reminder in the list

Reviewers: winterz, mlaurent

Reviewed By: mlaurent

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D16506