Cavegen: Rename CaveV6 to CavesV6

- Add comment explaining why it exists
- Remove unused 'flooded' variable
- Rename shadowed variable
- Fix some code style

src/mapgen_v6.cpp

@@ -1067,7 +1067,7 @@ void MapgenV6::generateCaves(int max_stone_y)
 
 	for (u32 i = 0; i < caves_count + bruises_count; i++) {
 		bool large_cave = (i >= caves_count);
-		CaveV6 cave(this, &ps, &ps2, large_cave);
+		CavesV6 cave(this, &ps, &ps2, large_cave);
 
 		cave.makeCave(node_min, node_max, max_stone_y);
 	}