Fix for memory leak in SessionManagerFixed

Summary:
An already by Inge Wallin marked memory leak is fixed by removing the
choosen test entries from the source list after copying them into the
destination list. So at the end of the choosing process only unused
entries remaining in the source list which are deleted before reusing
the list.

Depends on D17180

Reviewers: KDE Edu

Subscribers: tcanabrava, apol, kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D17201