Brighton, MA-based Caritas St. Elizabeth's Medical Center should abandon its role as a top-level academic hospital treating complex cases, and Caritas Carney Hospital in Dorchester should consider becoming a mental health center, according to a report on the Archdiocese of Boston's hospital chain by Attorney General Martha Coakley. The report also urges the archdiocese to cede control of the operation of Caritas Christi Health Care System so that an independent board with expertise in healthcare management can run it.