It appears that you want to delete a link. Somehow you need to choose which link that you want to delete.
Then you can either remove its link data object by calling GraphLinksModel.removeLinkData, or by calling Diagram.remove on the Link itself. Remember to do the removal within a single transaction.
You have to remove the actual link data object, the object that is in the GraphLinksModel.linkDataArray, not a copy that might happen to have some of the same properties.