[MIRROR] Clean up something else which was failing CI [MDB IGNORE] (#18372)

* Clean up something else which was failing CI (#72299)

## About The Pull Request

Well my last PR passed the CI but everything else is still failing and I
want a little tick next to my pull requests.
So I guess I'm whacking moles until everything that 515 doesn't know how
to dispose of is disposed of?

I'll be honest I don't know what change makes this a requirement when it
wasn't before but it seems like a logical thing to be doing.
Unfortunately running this test locally doesn't produce a failure for me
so my only solution is to keep getting the CI to do it and hope that it
doesn't report a false positive again.

## Why It's Good For The Game

![image](https://user-images.githubusercontent.com/7483112/209726114-4a6da427-f82b-433b-a7ef-39591dc72e7b.png)

## Changelog

Not player facing

* Clean up something else which was failing CI

* modular m90

Co-authored-by: Jacquerel <hnevard@gmail.com>
Co-authored-by: tastyfish <crazychris32@gmail.com>

code/modules/mining/minebot.dm

@@ -64,6 +64,7 @@
 	SetCollectBehavior()
 
 /mob/living/simple_animal/hostile/mining_drone/Destroy()
+	QDEL_NULL(stored_gun)
 	for (var/datum/action/innate/minedrone/action in actions)
 		qdel(action)
 	return ..()