Correct 4.2.1 release notes

Author: michaelklishin

release-notes/4.2.1.md

@@ -85,23 +85,6 @@ Release notes can be found on GitHub at [rabbitmq-server/release-notes](https://
    GitHub issue: [#14937](https://github.com/rabbitmq/rabbitmq-server/pull/14937)
 
 
-### HTTP Auth Backend Plugin
-
-#### Enhancements
-
- * The HTTP Auth Backend can now optionally provide a custom authorization denial reason to AMQP clients.
-
-   To opt in, return `deny <Reason>` (instead of only `deny`) in the HTTP response body of your HTTP auth backend and set the following in your `rabbitmq.conf` file:
-
-```ini
-auth_http.authorization_failure_disclosure = true
-```
-
-   See the [README](https://github.com/rabbitmq/rabbitmq-server/blob/main/deps/rabbitmq_auth_backend_http/README.md) for more information.
-
-   GitHub issue: [#14641](https://github.com/rabbitmq/rabbitmq-server/pull/14641)
-
-
 ### Dependency Changes
 
  * `cuttlefish` was upgraded to [`3.6.0`](https://github.com/kyorai/cuttlefish/releases)