Remove self-listener in conversation model

Since there is the only source/listener on this event, it can be called
directly.

// FREEBIE

js/models/conversations.js

@@ -46,7 +46,6 @@
 
         this.on('change:avatar', this.updateAvatarUrl);
         this.on('destroy', this.revokeAvatarUrl);
-        this.on('read', this.onReadMessage);
         this.fetchContacts().then(function() {
             this.contactCollection.each(function(contact) {
                 textsecure.storage.protocol.on('keychange:' + contact.id, function() {

js/read_receipts.js

@@ -35,7 +35,7 @@
 
                         if (conversation) {
                             // notify frontend listeners
-                            conversation.trigger('read', message);
+                            conversation.onReadMessage(message);
                         }
                     });
                 } else {