Remove copy of Akonadi's supertrait.h

Summary:
Instead, use the new way of annotating class hierarchies for Akonadi,
same as has been done in KCalCore. This should also fix the usage of
KMime::NewsArticle instances as Akonadi Item payloads.

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D21683