webhooks/github: Be more explicit about unsupported PR events.

This was technically a bug. For events that aren't unsupported
intentionally, the control should fall to the line that raises
UnknownWebhookEventType, and shouldn't be handled by anything else.

The events that are intentionally unsupported should be handled
more explicitly.

zerver/webhooks/github/view.py

@@ -389,8 +389,9 @@ def get_event(request: HttpRequest, payload: Dict[str, Any], branches: str) -> O
             return 'assigned_or_unassigned_pull_request'
         if action == 'closed':
             return 'closed_pull_request'
-        logging.warning(u'Event pull_request with {} action is unsupported'.format(action))
-        return None
+        # Unsupported pull_request events
+        if action in ('labeled', 'unlabeled', 'review_request_removed'):
+            return None
     if event == 'push':
         if is_commit_push_event(payload):
             if branches is not None: