(Hopefully) Fix Mind GC Issues (#2301)

Should fix weirdness with mob GC such as ghosts jumping to their deleted bodies, observers failing to GC, and mobs with minds not GCing in general.

changes:

Mob cleanup relating to the mob has been moved to the mind.
On deletion a mob will now null out the current and original fields in the mind, if they pointed to it.

code/modules/mob/mob.dm

@@ -11,8 +11,8 @@
 		for(var/atom/movable/AM in client.screen)
 			qdel(AM)
 		client.screen = list()
-	if(mind && mind.current == src)
-		spellremove(src)
+	if (mind)
+		mind.handle_mob_deletion(src)
 	for(var/infection in viruses)
 		qdel(infection)
 	viruses.Cut()