Drop unused dependencies KF5WindowSystem and KF5XmlGui
Summary: Depends on https://phabricator.kde.org/D6493
Test Plan: Builds fine.
Reviewers: Plasma, davidedmundson
Reviewed By: Plasma, davidedmundson
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D6494