Remove unused System Settings categories

Follow-up to powerdevil!214 (merged), and (newer revision with more categories) now also plasma-mobile!437 (merged).

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.