Fix memory leak when deleting MessageViewer and its NodeHelper.

Summary:
The Interface::BodyPartMemento(s) were not deleted.

Since they are deleted when switching messages, this is no severe
leak, it just showed up in a unittest.

Test Plan:
qDebug in ctor/dtor, from unittest.
Running kmail still works.

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: kde-pim, #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D2319