Do not leak all instances of QMLOutput when QMLScreen is destroyed

Summary:
In practice this is most likely only a leak on exit which does not
matter, but it makes using Valgrind etc impossible.

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: romangg, broulik, davidedmundson, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D14478