fix(ir.attachment.facturae.mx): remove unnecesary sql constrain for cfdi state
The original design for ir.attachment.facturae.mx object relay on a database constraint that prevents there were more than one object per state related to the same res_id. Unfortunately there is no such constrain on real life as could be possible that one customer invoice is related to more than one CFDI as could be required to cancel the CFDI several times and for history pourposes we need to keep a record for every CFDI that have being generated and cancelled. Already there is a function that works similar to a constraint and that prevents there are several validated CFDI's per invoice so I'm removing the unnecessary SQL constraint and reliyng on the function mentioned before.