Revert "Remove unregistered group members"

This reverts commit a768b94471.

d2ddfc7 was enough to fix #989. Removing unregistered members from the
group (as opposed to silently ignorning them) creates greater potential
for getting out of sync with the member lists on other devices.

// FREEBIE

js/models/conversations.js

@@ -62,19 +62,6 @@
         message.save().then(this.trigger.bind(this,'newmessage', message));
     },
 
-    addMemberLeft: function(source) {
-        var timestamp = Date.now();
-        var message = new Whisper.Message({
-            conversationId : this.id,
-            source         : source,
-            type           : 'incoming',
-            sent_at        : timestamp,
-            received_at    : timestamp,
-            group_update   : {left: source}
-        });
-        message.save().then(this.trigger.bind(this,'newmessage', message));
-    },
-
     onReadMessage: function(message) {
         if (this.messageCollection.get(message.id)) {
             this.messageCollection.get(message.id).fetch();