remove special case in World for InitOreDensity()

OpenRa.Game/Traits/OreGrowth.cs

@@ -16,7 +16,7 @@ namespace OpenRa.Traits
 		public object Create(Actor self) { return new OreGrowth(); }
 	}
 
-	class OreGrowth : ITick
+	class OreGrowth : ITick, ILoadWorldHook
 	{
 		int remainingTicks;
 
@@ -38,5 +38,10 @@ namespace OpenRa.Traits
 				remainingTicks = (int)(info.Interval * 60 * 25);
 			}
 		}
+
+		public void WorldLoaded(World w)
+		{
+			Ore.InitOreDensity(w.Map);
+		}
 	}
 }