[wayland] Send only clients that were on the removed desktop to a new desktop

Summary:
Currently, if a virtual desktop was removed, then we'll try to send all
clients to the last virtual desktop even though most of those clients
weren't present on the removed virtual desktop.

Test Plan: Manually.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D17576