Drop support for configuring private variables

[nex3]

See sass/sass#4034

• Deprecation ([Private Config] Implement the deprecation #2631)
• Removal
  • Specification ([With Private] Mark as obsolete sass#4190)
  • Dart Sass ([With Private] Enforce private variable configuration errors #2737)
  • Tests ([With Private] Enforce private variable configuration sass-spec#2112)
  • Documentation