Fixes a bunch of callbacks that were being qdeleted, and code cleanup (#77904)

## About The Pull Request


![image](https://github.com/tgstation/tgstation/assets/13398309/559eb50a-461c-4220-b628-55412baaffc3)

Continuing the work of
https://github.com/tgstation/tgstation/pull/77850.

it started with finding one that was being missed and causing a
runtime...then I noticed a whole lot more. While I was doing this I
found callbacks that weren't being nulled in `Destroy()`, so I added
that wherever I found these spots as well as some general code cleanup.

There were a lot more of these than I initially hoped to encounter so
I'm labeling it as a refactor.

## Why It's Good For The Game

Fixes lots of runtimes, improves code resiliency.

## Changelog

🆑
refactor: fixed a bunch of instances of callbacks being qdeleted and
cleaned up related code
/🆑

code/modules/admin/greyscale_modify_menu.dm

@@ -83,6 +83,8 @@
 /datum/greyscale_modify_menu/Destroy()
 	target = null
 	user = null
+	apply_callback = null
+	config = null
 	return ..()
 
 /datum/greyscale_modify_menu/ui_state(mob/user)