Simplify FavoriteCollectionsModel: inherit KSelectionProxyModel

Summary:
instead of inheriting from Akonadi::SelectionProxyModel and disabling
the very little that Akonadi::SelectionProxyModel does compared to
its base class, KSelectionProxyModel.

And add a comment about why QSortFilterProxyModel wouldn't do,
replacing the comment that we should inherit from that instead.

Test Plan: Favorites work like before. The code is cleaner :)

Reviewers: dvratil, cmollekopf

Reviewed By: dvratil

Subscribers: KDE PIM

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D8802