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/displaycase.dm

@@ -47,13 +47,13 @@
 	if(vname in list(NAMEOF(src, open), NAMEOF(src, showpiece), NAMEOF(src, custom_glass_overlay)))
 		update_appearance()
 
-/obj/structure/displaycase/handle_atom_del(atom/A)
-	if(A == electronics)
+/obj/structure/displaycase/Exited(atom/movable/gone, direction)
+	. = ..()
+	if(gone == electronics)
 		electronics = null
-	if(A == showpiece)
+	if(gone == showpiece)
 		showpiece = null
 		update_appearance()
-	return ..()
 
 /obj/structure/displaycase/Destroy()
 	QDEL_NULL(electronics)
@@ -73,8 +73,6 @@
 	if(QDELETED(showpiece))
 		return
 	showpiece.forceMove(drop_location())
-	showpiece = null
-	update_appearance()
 
 /obj/structure/displaycase/play_attack_sound(damage_amount, damage_type = BRUTE, damage_flag = 0)
 	switch(damage_type)