Removes (now unused) sandbox related config and variable (#66803)

I don't know when we removed sandbox but this datum doesn't exist anymore and this var is unused

Also the config does nothing anymore so might as well throw that out

code/modules/mob/mob_defines.dm

@@ -221,8 +221,6 @@
 	var/list/client_colours = list()
 	var/hud_type = /datum/hud
 
-	var/datum/h_sandbox/sandbox = null
-
 	var/datum/focus //What receives our keyboard inputs. src by default
 
 	/// Used for tracking last uses of emotes for cooldown purposes