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: