[Notifier] Cleanup changelog (5.1)

Author: OskarStark

CHANGELOG.md

@@ -4,7 +4,6 @@ CHANGELOG
 5.1.0
 -----
 
- * Added the Mattermost notifier bridge
  * [BC BREAK] The `ChatMessage::fromNotification()` method's `$recipient` and `$transport`
    arguments were removed.
  * [BC BREAK] The `EmailMessage::fromNotification()` and `SmsMessage::fromNotification()`