A follow-up to rP21975:
- Removes <Civ>gaia</Civ> from the simulation/templates/other/ files because this is already present in template_structure.xml, from which these inherit (I don't remember why these other/ files were not included in D1645)
- Also adds <Civ>gaia</Civ> to template_structure_defense_wallset.xml, because confusingly this file does *not* have a parent