Remove orphan single sources
In some classes only a single source is referenced (e.g. classes implementing SingleSourcedEntityBase or TaxonName.nomenclaturalSource or TaxonNode.source).
With TaxonNode.source it was tested if @OneToOne mapping with orphanRemoval=true (and Cascade.PERSIST) works but it did not. Some research needs to be done how this can be solved. However, it is not critical as only orphaned sources are created and these cases are relatively rare. So this is not a requirement for #9004 and #6581