Use qDeleteAll in destructor of Scene class

Summary:
qDeleteAll can be used to delete values in QMap or QHash. Please notice
that qDeleteAll calls `delete` only for values, not keys.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D13822