Mapgen: Optimise cave noises and tunnel excavation

Instead of doing nothing at node_max.Y + 1 use 1-down
overgeneration for tunnel generation and noisemaps
Move some old unused code in mgv7 to end of file

src/mapgen_valleys.h

@@ -106,6 +106,7 @@ private:
 
 	int ystride;
 	int zstride;
+	int zstride_1d;
 
 	float map_gen_limit;