[tabbox] Drop TabBoxClient::window() method
Summary: TabBoxClient::window() method is no longer used by the tabbox.
Test Plan: Compiles, tests still pass.
Reviewers: KWin, davidedmundson
Reviewed By: KWin, davidedmundson
Subscribers: kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D25092