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