Remove outdated PackageKit include

Summary:
PackageKitQt5_FOUND can never be set, it's a leftover that should be
removed wtih 27d71ef5

Reviewers: #plasma, hein, apol

Reviewed By: #plasma, hein, apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D3921