how can I tear down these components and their relationship to an entity id? Do I just look over all subsystems that are in the subsystem registry and notify them that entity ABC should be removed?
That's the easy way. And in fact entity death is infrequent enough that you should probably start with that, and you'll never find a reason to change it.
If you decide that you can't stomach potentially wasted searches in subsystems which don't happen to have a component for the entity in question, then you need a record of which components make up an entity. That could be maintained in an actual entity object.
Or if you want to get clever(?) you could go ahead and make the call on all sub-systems, but have them cache a list of entities to remove, and perform the removal during their normal processing of their components (you might have to do this sort of thing anyway, if you're multi-threading). That eliminates the extra searches. And that list will usually be empty, and rarely have more than one entry.