One more hard delete (shiptest-ss13#2790)

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## About The Pull Request

missed one relating to missions

## Why It's Good For The Game

harddel bad

## Changelog

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/:cl:

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code/modules/overmap/missions.dm

@@ -59,7 +59,7 @@
 		servant = null
 	for(var/bound in bound_atoms)
 		remove_bound(bound)
-	dur_timer = null
+	deltimer(dur_timer)
 	return ..()
 
 /datum/mission/proc/turn_in()