Crate shelvers no longer persist after attempting to spawn (#4035)

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## About The Pull Request
`mapping_helpers/crate_shelve` now qdels after attempting to shelve a
crate, meaning observers won't see leftover helpers.
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## Why It's Good For The Game
Don't see mapping helpers as a ghost
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## Changelog

:cl:
fix: the outpost's crate shelvers have been trained to leave no evidence
/:cl:

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code/modules/mapping/mapping_helpers.dm

@@ -503,6 +503,7 @@ INITIALIZE_IMMEDIATE(/obj/effect/mapping_helpers/no_lava)
 		log_mapping("[src] failed to find a crate at [AREACOORD(src)]")
 	else
 		shelve(crate)
+	qdel(src)
 
 /obj/effect/mapping_helpers/crate_shelve/proc/shelve(crate)
 	var/obj/structure/crate_shelf/shelf = locate(/obj/structure/crate_shelf) in range(range, crate)