revert the hackish overriding of Client::desktop() for session saving

The member variable is not what determines which desktop a window is on,
it's more like caching the state. Therefore trying to fake a new value
eventually leads to inconsistencies, e.g. having KWrite open with unsaved
changes on inactive desktop results in the window ending up on all desktops
after session save (both successful and cancelled).
This pretty much reverts the whole 7ce38049 that introduced this and also
a0a97688 that tried to fix some of the problems resulting from it.
The original problem of session saving of windows of inactive activities still
remains, to be fixed by another commit.

Differential Revision: https://phabricator.kde.org/D5613