OpenDream Cleanup Pass - Unused Vars (#71428)

## About The Pull Request

OpenDream (@Altoids1 specifically) discovered that BYOND will not throw
an unused variable warning if its type doesn't exist. So this removes
those from TG.

Co-authored-by: ike709 <ike709@github.com>

code/modules/mob/dead/observer/observer.dm

@@ -23,7 +23,6 @@ GLOBAL_VAR_INIT(observer_default_invisibility, INVISIBILITY_OBSERVER)
 	light_on = FALSE
 	shift_to_open_context_menu = FALSE
 	var/can_reenter_corpse
-	var/datum/hud/living/carbon/hud = null // hud
 	var/bootime = 0
 	var/started_as_observer //This variable is set to 1 when you enter the game as an observer.
 							//If you died in the game and are a ghost - this will remain as null.