drop the dead payloads from the state-change events

src/lib/eventsystem/event/ExpansionCostChangedEvent.h

@@ -4,14 +4,11 @@
 #include "Event.h"
 
 // Fired when the current asteroid-expansion cost changes (REQ-EXP-COST): once at
-// startup and again after each expansion is purchased. Carries the cost in
-// building blocks so the header Expand button can update its caption/enabled
-// state (REQ-UI-EXPAND-BUTTON).
+// startup and again after each expansion is purchased. Carries no payload — the
+// header Expand button re-reads Simulation::getCurrentExpansionCost() to update
+// its caption and enabled state (REQ-UI-EXPAND-BUTTON).
 class ExpansionCostChangedEvent : public Event
 {
-public:
-	explicit ExpansionCostChangedEvent(int cost) : cost(cost) {}
-	const int cost;
 };
 
 #endif   // EXPANSION_COST_CHANGED_EVENT_H