Simplify transient initialization in Deleted

Summary:
AbstractClient::mainClients is quite generic, it can return either
transientFor parent or group member parents.

Test Plan: Existing tests still pass.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D16586