webhooks/librato: Stop advertising Snapshot notifications.

We ask our users to enable Snapshot notifications in Zulip via
Slack! But our Slack integration isn't exactly super robust and
I checked and our librato implementation isn't super smart about
handling snapshot payloads that come in via Slack.

Overall, this seems like a very poor solution, asking the user
to set up Slack in order to get the notifications in Zulip. So, I
thought we should get rid of at least the docs that suggest doing
this.

I also read librato/view.py and it wasn't clear to me how Slack
is supposed to act as an intermediate service here in a reliable
manner, which is another reason to not advertise this.

zerver/webhooks/librato/doc.md

@@ -1,6 +1,4 @@
-Get Zulip notifications for your Librato/AppOptics alerts or snapshots!
-
-### Set up notifications for Alerts
+Get Zulip notifications for your Librato/AppOptics alerts!
 
 1. {!create-stream.md!}
 
@@ -19,20 +17,3 @@ Get Zulip notifications for your Librato/AppOptics alerts or snapshots!
 {!congrats.md!}
 
 ![](/static/images/integrations/librato/001.png)
-
-### Set up notifications for Snapshots
-
-Because of limitations in Librato's API, you need to use the
-[Slack integration](./slack) to get Librato snapshots sent into Zulip.
-
-The default stream name is **librato** and default topic name is
-**snapshots**.
-
-To send a snapshot, just click on one of your charts, use
-the **send a snapshot** option and add the proper integration.
-
-![](/static/images/integrations/librato/008.png)
-
-{!congrats.md!}
-
-![](/static/images/integrations/librato/009.png)