antag_datums GC fixes (#18321)

code/modules/antagonists/_common/antag_datum.dm

@@ -13,8 +13,6 @@ GLOBAL_LIST_EMPTY(antagonists)
 	var/silent = FALSE
 	/// List of other antag datum types that this type can't coexist with.
 	var/list/antag_datum_blacklist
-	/// Should this datum be deleted when the owner's mind is deleted.
-	var/delete_on_mind_deletion = TRUE
 	/// Used to determine if the player jobbanned from this role. Things like `SPECIAL_ROLE_TRAITOR` should go here to determine the role.
 	var/job_rank
 	/// Should we replace the role-banned player with a ghost?