Dunking handle_atom_del() in the trash bin. (#77339)

Whatever you do, if it warrants the use of something like
`handle_atom_del`, chances are `Exited` can do it better, as most of
these cases involve movables that shouldn't be moved out of their loc
(`Destroy` forcefully moves movables to nullspace) without calling
specific procs, and for the remaining few, `handle_atom_del` doesn't
even cover the eventuality of a movable being deleted outside the source
atom, so it's quite garbage.

Beside, I feel confident in saying `handle_atom_del()` is older than the
DCS, an echo on the workarounds done at the time.

code/game/objects/structures/guncase.dm

@@ -92,7 +92,6 @@
 		return
 	if(!user.put_in_hands(weapon))
 		weapon.forceMove(get_turf(src))
-	update_appearance()
 
 /**
  * check_menu: Checks if we are allowed to interact with a radial menu
@@ -109,7 +108,8 @@
 		return FALSE
 	return TRUE
 
-/obj/structure/guncase/handle_atom_del(atom/A)
+/obj/structure/guncase/Exited(atom/movable/gone, direction)
+	. = ..()
 	update_appearance()
 
 /obj/structure/guncase/contents_explosion(severity, target)