webhooks: Remove the Facebook integration.

Rishi and I decided that it makes sense to get rid of the Facebook
integration for a few reasons, some of which are:

* The setup process is too complicated on Facebook's end. The users
  will surely have to browse Facebook's huge API reference before even
  having a vague idea of what they want.
* Slack chooses not to have a Facebook integration, but relies on
  Zapier for it. Zaps that integrate with Facebook are much more
  streamlined and the setup process isn't as much of a pain. Zapier's
  Facebook Zaps are much more fine-tuned and there are different Zaps
  for different parts of the FB API, a luxury that would likely span
  2K+ lines of code on our end if we were to implement it from
  scratch. So, I think we should relegate integration with Facebook to
  Zapier as well!
* After thoroughly testing the setup process, we concluded that the
  person who submitted the FB integration didn't really test it
  thoroughly because there were some gaping holes in the docs (missing
  steps, user permissions, etc.).

zerver/lib/integrations.py

@@ -373,7 +373,6 @@ WEBHOOK_INTEGRATIONS = [
     WebhookIntegration('zendesk', ['customer-support']),
     WebhookIntegration('gci', ['misc'], display_name='Google Code-in',
                        stream_name='gci'),
-    WebhookIntegration('facebook', ['communication'], display_name='Facebook')
 ]  # type: List[WebhookIntegration]
 
 INTEGRATIONS = {