title: django-admin makemessages --no-obsolete doesn't seem to be working
body: <p>First of all, I am expecting <code>--no-obsolete</code> would comment out <code>msgid</code> and <code>msgstr</code> if <code>gettext</code> is deleted, right?</p>
  
  <p>How I am testing is:</p>
  
  <ol>
  <li>I wrote <code>gettext("some string here")</code> in view  </li>
  <li>I ran <code>makemessages</code> command  </li>
  <li>It wrote a <code>.po</code> file as expected  </li>
  <li>Then I deleted <code>gettext()</code> line from view and saved file, verified <code>runserver</code> working.  </li>
  <li>I ran <code>makemessages --no-obsolete</code> and it has not made any changes to <code>.po</code> file.  </li>
  </ol>
  
  <p><code>.po</code> file content extract . </p>
  
  <pre><code>#. Translators: This message is a test of wrap line
  #: servers/views.py:31
  msgid "Do let me know if it works."
  msgstr ""
  </code></pre>
  
  <p><strong>dev environment</strong></p>
  
  <blockquote>
    <p>Django =  1.11<br>
    OS = Mac/Ubuntu 14.04</p>
  </blockquote>
  
  <p><strong>settings.py</strong></p>
  
  <pre><code>LANGUAGE_CODE = 'en-us'
  
  TIME_ZONE = 'UTC'
  
  USE_I18N = True
  
  USE_L10N = True
  
  USE_TZ = True
  
  LOCALE = (
        os.path.join(os.path.dirname(__file__), "locale"),
  )
  </code></pre>