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