Remove no longer needed cast to Client in Workspace::sendClientToDesktop
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
Differential Revision: https://phabricator.kde.org/D17546