Remove unused System Settings categories

All known KCMs in these categories have been reparented for Plasma 6:
* power-management (previously part of system-administration)
* connectivity (previously part of network)
* desktopbehavior (previously a top-level category)
* shortcuts (previously part of workspace)

Any third-party KCMs that we missed, but were using one of the above
as parent category, should be reparented as well.


(cherry picked from commit f3258fa6)