Remove no longer needed cast to Client in Workspace::sendClientToDesktop

Summary:
This change removes a small difference between X11 and Wayland clients.
It ensures that all transients are sent to the same desktop as the main
window. A similar check is already in AbstractClient::setDesktop, so in
general it already worked. This is just a special case for
sendClientToDesktop which supports sending to the same desktop so that
all transients are sent to that desktop.

Test Plan: New test case which fails without this change

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D17546