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 #7186 (comment)

PR for v9: #7671

[atsareg]

The instructions are added and a script is available. The script is still to be merged (possibly) with the one which is updating the Operations section for the same purpose (dropping Setups). The script needs minor modifications to cope with possible non-System sections in the /Systems.