Removed double newline from output of migrate --list

refs [#23359](https://code.djangoproject.com/ticket/23359) and PR https://github.com/django/django/pull/3153#issuecomment-68471839
This will be redundant when https://code.djangoproject.com/ticket/24068 is fixed, right? If yes, it would be good to add a comment about the issue.