Now deletes previous xrefs from the same source.
Now deletes previous xrefs from the same source.

Refactored.

at7/ensembl,Ensembl/ensembl,mjg17/ensembl,muffato/ensembl,danstaines/ensembl,Ensembl/ensembl,danstaines/ensembl,at7/ensembl,willmclaren/ensembl,james-monkeyshines/ensembl,at7/ensembl,james-monkeyshines/ensembl,mjg17/ensembl,james-monkeyshines/ensembl,muffato/ensembl,muffato/ensembl,mjg17/ensembl,at7/ensembl,muffato/ensembl,willmclaren/ensembl,Ensembl/ensembl,james-monkeyshines/ensembl,Ensembl/ensembl,mjg17/ensembl,danstaines/ensembl