Apply relationship inference logic to deletions.

[lucas42]

At the moment, deleting a relationship only deletes that specific instance. (eg deleting a sibling relationship from Alice to Bob will still leave the relationship from Bob to Alice in place).

Ideally, the inference logic would go through all the relationships which imply the deleted relationship should exist and delete those to prevent inconsistencies.

Complexity
Whilst this is probably easy enough for inverses (and transitive?), it's hard to know what to do for multi-relation inferences.
Suppose a "nibling" relationship is deleted. This suggests that either a corresponding "sibling" relationship or "child" relationship is incorrect. However, there's no way to know for certain which of these to delete, as the other one may be perfectly valid.