Update conversation lastMessage from database

Don't set lastMessage, let it update itself as needed, such as when
first rendering a conversation list item, and when its messages are
sent, received, or destroyed.

js/models/messages.js

@@ -424,10 +424,6 @@
                                 timestamp: message.get('sent_at')
                             });
                         }
-                        conversation.set({
-                            lastMessage: message.getNotificationText()
-                        });
-
                         message.save().then(function() {
                             conversation.save().then(function() {
                                 conversation.trigger('newmessage', message);