2012/644/EU: Council Decision of 24 September 2012 on the signing, on behalf of the European Union, of the Agreement in the form of an Exchange of Letters between the European Union and the United States of America pursuant to Article XXIV:6 and Article XXVIII of the General Agreement on Tariffs and Trade (GATT) 1994 relating to the modification of concessions in the schedules of the Republic of Bulgaria and Romania in the course of their accession to the European Union