Merge pull request #9457 from variableundefined/30Mimes

Fixes math error with mime forcewall

code/game/objects/effects/forcefields.dm

@@ -17,16 +17,9 @@
 	icon_state = "empty"
 	name = "invisible wall"
 	desc = "You have a bad feeling about this."
-	var/timeleft = 300
-	var/last_process = 0
+	var/lifetime = 30 SECONDS
 
 /obj/effect/forcefield/mime/New()
 	..()
-	last_process = world.time
-	processing_objects.Add(src)
-
-/obj/effect/forcefield/mime/process()
-	timeleft -= (world.time - last_process)
-	if(timeleft <= 0)
-		processing_objects.Remove(src)
-		qdel(src)
+	if(lifetime)
+		QDEL_IN(src, lifetime)