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