Temporarily remove user-visible "time travel" UI.

I'd like to think about how to polish aspects of this a bit more,
but would like us to be able to deploy master at some point today.

This is basically the philosophical equivalent of reverting the
user-visible UI changes introduced in
b7b6794ad635ec63269a2043cd48b02749fbffda

(imported from commit edfaadf26741c47120c3acf6c410d33025c0a260)

zephyr/jstemplates/message.html

@@ -58,7 +58,7 @@
             <span class="sender_email invisible">{{sender_email}}</span>
         </span>
     {{/include_sender}}
-        <span class="message_time user_info_hover"
+        <span class="message_time"
               title="{{full_date_str}}">{{{timestr}}}</span>
         <div class="message_content">{{{content}}}</div>
     </td>

zephyr/jstemplates/userinfo_popover_content.html

@@ -12,9 +12,4 @@
     </a>
   </li>
   {{/unless}}
-  <li>
-    <a onclick="ui.hide_userinfo_popover(); narrow.target({{id}}); narrow.time_travel();">
-      <i class="icon-time"></i> Jump to this time and view all messages
-    </a>
-  </li>
 </ul>