Fix clearing PageRow

Summary:
Makes sense because we grow in depth, so it makes sense that we free the
resources in the same order.
Clear from end to beginning rather than the other way around. It would cause
crashes in Discover (within QtQuick).

Reviewers: Kirigami, mart

Reviewed By: mart

Subscribers: plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D3795