Remove usage of KDBusConnectionPool

Summary:
As per T12722 this is no longer needed, QDBusConnection now behaves
correctly in a multi-threaded scenario.

Reviewers: #kde_pim, mlaurent

Reviewed By: #kde_pim, mlaurent

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D27551