remove m_desktopContainments
Summary:
with new corona containment query functions in D11361 the data
copy done in m_desktopContainments is no more necessary: this should
make things a bit more simple and robust
Test Plan: launched plasmashell, tried to add/remove activities and screens, all keeps working
Reviewers: Plasma, davidedmundson
Reviewed By: Plasma, davidedmundson
Subscribers: davidedmundson, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D11801