Medical vendor Create&Destroy fix (#7222)

# About the pull request

Medical vendors are added to the slowobj subsystem but not removed on
Destroy(). This fixes that.

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# Explain why it's good for the game

Lint fails bad.
# Testing Photographs and Procedure
<details>
<summary>Screenshots & Videos</summary>

Before, from PvE:

![image](https://github.com/user-attachments/assets/c585c054-7e87-45ec-82e3-b4e9078efaad)

![image](https://github.com/user-attachments/assets/d5737ead-4370-4c01-994e-87471f80ed38)

</details>


# Changelog
No player facing changes.

code/game/machinery/vending/vendor_types/medical.dm

@@ -268,6 +268,7 @@
 	)
 
 /obj/structure/machinery/cm_vending/sorted/medical/Destroy()
+	STOP_PROCESSING(SSslowobj, src)
 	QDEL_NULL(last_health_display)
 	. = ..()