Don't load system apps model twice
Summary:
The system apps model is set from QML via
ComputerModel::setSystemApplications
This is always called from Kickoff, and Kickoff is the only user of this
class.
Reviewers: mart, Plasma, hein
Reviewed By: mart, Plasma, hein
Subscribers: hein, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D2841