Remove usage of KDBusConnectionPool
Summary:
As per T12722 this is no longer needed, QDBusConnection now behaves
correctly in a multi-threaded scenario.
This doesn't remove all traces of KDBusAddons yet, the remaining bits
need the users of Akonadi CMake macros to be ported first.
Reviewers: dvratil
Reviewed By: dvratil
Subscribers: kde-pim
Tags: KDE PIM
Differential Revision: https://phabricator.kde.org/D27510