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