Makes playable mobs GC better by reworking how GLOB.respawnable_list works (#17217)

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

@@ -6,4 +6,4 @@
 
 	if(GLOB.non_respawnable_keys[ckey])
 		can_reenter_corpse = 0
-		GLOB.respawnable_list -= src
+		remove_from_respawnable_list()

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

@@ -164,9 +164,9 @@ Works together with spawning an observer, noted above.
 			flags &= ~GHOST_CAN_REENTER
 		var/mob/dead/observer/ghost = new(src, flags)	//Transfer safety to observer spawning proc.
 		ghost.timeofdeath = src.timeofdeath //BS12 EDIT
-		GLOB.respawnable_list -= src
+		remove_from_respawnable_list()
 		if(ghost.can_reenter_corpse)
-			GLOB.respawnable_list += ghost
+			ghost.add_to_respawnable_list()
 		else
 			GLOB.non_respawnable_keys[ckey] = 1
 		ghost.key = key