This patch cleans up some template_structure* lines.
Split off from D2855:
- Defines attack sounds only in those structure templates that define an <Attack>.
Split off from D2993:
- Defines <BuildingAI> only in those structure templates that define an <Attack>.
Split off from D3000:
- Removes <Sound> from template_structure_special.xml, since most of its children have different values.
- Removes <Health/Max> from template_structure_special.xml, since that's already present in most its children.
- Removes <TerritoryInfluence> from template_structure_special.xml, since that belongs in most of its children too.
- Purges unused <BuildRestrictions/Category> lines.