Reduce the amount of mobspawners in trashpiles by 50% (#6200)

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## About The Pull Request
makes it so that trashpiles flip a coin on whether or not to have a
mouse spawner.
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## Why It's Good For The Game
Honestly mice are hardly any use in game, aside from the rare character
eating them, and the occasional griefer throwing tonnes of them into the
bar after storing them in backpacks.
Additional I dislike the constant squeeking trying to walk through
maint.
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:cl:
tweak: tweaked trash piles to not always spawn mice
/:cl:

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code/game/objects/structures/trash_pile.dm

@@ -43,12 +43,9 @@
 		"boxfort",
 		"trashbag",
 		"brokecomp")
-	pest_nest = new(src)
+	if(prob(50))
+		pest_nest = new(src)
 
-/obj/structure/trash_pile/Destroy()
-	qdel(pest_nest)
-	pest_nest = null
-	return ..()
 
 /obj/structure/trash_pile/attackby(obj/item/W as obj, mob/user as mob)
 	var/w_type = W.type
@@ -124,6 +121,12 @@
 	else
 		return ..()
 
+/obj/structure/trash_pile/proc/create_mob_spawner()
+	if(!pest_nest)
+		pest_nest = new(src)
+	else
+		debug_admins("[src] tried to create pest_nest when pest_nest already existsed([pest_nest])")
+
 //Random lists
 /obj/structure/trash_pile/proc/produce_alpha_item()
 	var/path = pick(prob(5);/obj/item/clothing/gloves/rainbow,