[FIX] mail: Don't display message if only subtype

Currently, if a message does not have any tracking
value, any attachment and no body BUT has a subtype
with a description, the message is not considered
as empty.

The message subtype should not influence its
"emptyness".

addons/mail/static/src/js/models/messages/abstract_message.js

@@ -255,7 +255,6 @@ var AbstractMessage =  Class.extend({
      */
     isEmpty: function () {
         return !this.hasTrackingValues() &&
-        !this.hasSubtypeDescription() &&
         !this.hasAttachments() &&
         !this.getBody();
     },