Removal of "Systems" part of CS

[fstagni]

• this PR can be merged, deployed, and nothing should happen
• at some point in time, the admins should:
  • set the DIRAC/NoSetup=True AND
  • remove the Systems part of CS to remove the instance parts
  • the /DIRAC/Setups part of CS can be entirely removed
• at this point update to v9 can happen "freely"

At a minimum we need some instructions, to be added to the v9 wiki. A script that does the CS update would be better.

Originally posted by @fstagni in https://github.com/DIRACGrid/DIRAC/issues/7186#issuecomment-2360653595