I recently encountered a situation where I needed to create a collection without a Limiting Collection, which was an interesting challenge. What I found is that for some built-in collections, including All Systems, the Limiting Collection is NULL in database, which you can see with this query.
Select SiteID, LimitToCollectionID, CollectionName from collections_g where CollectionName = 'All Systems'
With that information, I found you can remove the Limiting Collection from any collection you create with this statement.
Update collections_g set LimitToCollectionID = NULL where siteid = 'CollectionID'
Now, just because you can do this, doesn't mean it is a good idea. First and foremost, without a Limiting Collection, anything can be included in the collection, so you need to be careful with your rules to ensure that you are only selecting the objects that should be included. Also, once you remove the Limiting Collection, the collection becomes read only after a Membership Update runs, so again it is important to make sure that your collection rules are correct.
Lastly, why would you want to create a collection without a limiting collection? Check out this post to find out!