Replace explicit z-level checks with defines

code/modules/procedural_mapping/mapGenerators/lava_river.dm

@@ -1,5 +1,5 @@
 /datum/mapGenerator/lavaland
-	var/start_z = 5
+	var/start_z = ZLEVEL_LAVALAND
 	var/min_x = 0
 	var/min_y = 0
 	var/max_x = 0
@@ -18,7 +18,7 @@
 /datum/mapGeneratorModule/river
 	var/river_type = /turf/open/lava/smooth
 	var/river_nodes = 4
-	var/start_z = 5
+	var/start_z = ZLEVEL_LAVALAND
 
 /datum/mapGeneratorModule/river/generate()
 	var/datum/mapGenerator/lavaland/L = mother