Start KOrgac via DBus activation

Summary:
KOrgac is already autostarted on session start, but KOrganizer had
some additional code to start it. Instead of using needlesly complicated
code in KdepimDBusInterfaces, simply use D-Bus service activation.

Reviewers: #kde_pim, vkrause

Reviewed By: #kde_pim, vkrause

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D29495