Removed fake restriction units and refactored flood plains legacy code

[HaneulCheong]

• Removed city-state fake restriction units
  • Fake units for Arroyo and Ghost Mountain are removed and replaced with national uniques.
  • All fake units are now removed as far as I can tell.
• Restructured Units.json
  • All entries are now under somewhat logical categories marked with comments.
  • Moved around some code, but the entries are completely the same as before.
• Updated deprecated uniques
  • "Requires [Stable]" (\~3.19.11) -> "Only available <if [Stable] is constructed>" (3.19.12\~)
• Partially removed and refactored Flood plains legacy code
  • Should probably remove everything after a few more days (or a couple of weeks?).