Remove extraneous comment from Action Cable overview guide [ci skip]

This section isn't about web notifications, there's an example section
later on in the guide that talks about that and has a similar comment on
it, here it was unnecessary / confusing.

guides/source/action_cable_overview.md

@@ -373,7 +373,6 @@ these channel subscriptions based on an identifier sent by the cable consumer.
 
 ```js
 // app/javascript/channels/chat_channel.js
-// Assumes you've already requested the right to send web notifications
 import consumer from "./consumer"
 
 consumer.subscriptions.create({ channel: "ChatChannel", room: "Best Room" }, {