[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