loginDialog: Don't arbitrarily pick a random session for the user

gdm should be in charge of telling us the default session, and it
should pick one for the user if she doesn't already have one.

https://bugzilla.gnome.org/show_bug.cgi?id=740142

js/gdm/loginDialog.js

@@ -354,9 +354,6 @@ const SessionMenuButton = new Lang.Class({
             this._menu.addMenuItem(item);
             this._items[id] = item;
 
-            if (!this._activeSessionId)
-                this.setActiveSession(id);
-
             item.connect('activate', Lang.bind(this, function() {
                 this.setActiveSession(id);
             }));