Stop the chat app listening to 'talkilla.spa-channel-message' as the SPAChannel already does this.

`talkilla.spa-channel-message` is currently handled twice in the chat window: firstly by the `SPAChannel`, secondly by `ChatApp#_onIncomingTextConversation`.

This second one is creating/replacing the transport, as well as triggering on the new message.

The end result is we're recreating the transport each time, as well as triggering the message notification twice.

We can just remove `ChatApp#_onIncomingTextConversation` and let `SPAChannel` handle it.