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_singlenode.h

@@ -38,7 +38,7 @@ class MapgenSinglenode : public Mapgen
 	content_t c_node;
 	u8 set_light;
 
-	MapgenSinglenode(int mapgenid, MapgenParams *params, EmergeManager *emerge);
+	MapgenSinglenode(MapgenParams *params, EmergeManager *emerge);
 	~MapgenSinglenode() = default;
 
 	virtual MapgenType getType() const { return MAPGEN_SINGLENODE; }