Remove setting for per-activity virtual desktop remembering

I'm moving this functionality into kwin to fix bug BUG: 427579. In the process, I'm removing this option (it will now always be enabled) in preparation to make each activity have it's own unique set of virtual desktops (activities and desktops will stop being orthogonal). When this happens, (and one VD is never in multiple activities), having this option won't make any sense. @teams/vdg @teams/usability

I think I've removed all the relevant code.

Related MRs: kwin!3011 kactivitymanagerd!33