Delete unused function

Summary:
The function was introduced in 2d99ef918bbd797701ac63c8e3d4d7ca05f64651.

It's not used anywhere plus we don't need it anymore because
AbstractClient::isActiveFullscreen checks main clients.

Test Plan: Compiles.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D16022