cleanup: Don't shadow variables

Having variables that share the same name in overlapping scopes is
confusing and error-prone, and is best avoided.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/805

js/gdm/loginDialog.js

@@ -984,7 +984,7 @@ var LoginDialog = GObject.registerClass({
         let hold = new Batch.Hold();
         let signalId = this._userList.connect('item-added',
             () => {
-                let item = this._userList.getItemFromUserName(userName);
+                item = this._userList.getItemFromUserName(userName);
 
                 if (item)
                     hold.release();