mapgen: drop mapgen id from child mapgens.

This id must be owned by the child mapgen and never be set to a misc value by a developer

Also use nullptr in some places

src/mapgen/mapgen_valleys.h

@@ -79,7 +79,7 @@ struct MapgenValleysParams : public MapgenParams {
 class MapgenValleys : public MapgenBasic {
 public:
 
-	MapgenValleys(int mapgenid, MapgenValleysParams *params,
+	MapgenValleys(MapgenValleysParams *params,
 		EmergeManager *emerge);
 	~MapgenValleys();