Remove local copy of KCalCore::DateList typedef

Summary:
We get the real one from the KCalCore/Incidence include above anyway,
and this way this code will automatically adjust to the SortableList
removal.

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D20308