Slash SSatmos init time by 93%

code/controllers/subsystem/air.dm

@@ -288,9 +288,12 @@ SUBSYSTEM_DEF(air)
 	var/list/active_turfs = src.active_turfs
 	var/times_fired = ++src.times_fired
 
+	// Clear active turfs - faster than removing every single turf in the world
+	// one-by-one, and Initalize_Atmos only ever adds `src` back in.
+	active_turfs.Cut()
+
 	for(var/thing in turfs_to_init)
 		var/turf/T = thing
-		active_turfs -= T
 		if (T.blocks_air)
 			continue
 		T.Initalize_Atmos(times_fired)