Remove outdated PackageKit include

Summary:
PackageKitQt5_FOUND can never be set, it's a leftover that should be
removed wtih 27d71ef5f394f6a53aff3e022ae0145e09e89765

Reviewers: Plasma, hein, apol

Reviewed By: Plasma, hein, apol

Subscribers: plasma-devel

Tags: Plasma

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