Refactor weather to use Z traits, assorted related cleanup (#34633)

* Add a proc for getting the station center

* Add a couple of comments to ZTRAIT defines

* Remove unused global_map list

* Refactor weather to use the trait system

* Un-hardcode the transit z-level

* Use Z traits to determine Portal Storm event areas

* Fix loading away missions containing anything that reads traits

code/__HELPERS/unsorted.dm

@@ -1199,19 +1199,6 @@ B --><-- A
 	sleep(duration)
 	A.cut_overlay(O)
 
-/proc/get_areas_in_z(zlevel)
-	. = list()
-	var/validarea = FALSE
-	for(var/V in GLOB.sortedAreas)
-		var/area/A = V
-		validarea = TRUE
-		for(var/turf/T in A)
-			if(T.z != zlevel)
-				validarea = FALSE
-				break
-		if(validarea)
-			. += A
-
 /proc/get_closest_atom(type, list, source)
 	var/closest_atom
 	var/closest_distance