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