remove --duplicates: when no duplicates, exit with 0 (RHEL-6424)

If no duplicates are present, then the command succesfully removed all
duplicates and should exit with 0 and write the message to stdout
instead of stderr.

Resolves: https://issues.redhat.com/browse/RHEL-6424

dnf/cli/commands/remove.py

@@ -92,7 +92,8 @@ def run(self):
             instonly = self.base._get_installonly_query(q.installed())
             dups = q.duplicated().difference(instonly)
             if not dups:
-                raise dnf.exceptions.Error(_('No duplicated packages found for removal.'))
+                logger.info(_('No duplicated packages found for removal.'))
+                return
 
             for (name, arch), pkgs_list in dups._na_dict().items():
                 if len(pkgs_list) < 2: