Remove feature flag documentation details for batch_review_notification

This reverts merge request
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8933

doc/user/discussions/index.md

@@ -343,10 +343,6 @@ merge request with all the comments associated to it.
 
 Replying to this email will, consequentially, create a new comment on the associated merge request.
 
-The review email is currently hidden behind the `batch_review_notification` feature flag,
-and can be enabled by contacting your GitLab instance administrator and having them
-run `Feature.enable(:batch_review_notification)` using the `gitlab-rails console`.
-
 ## Filtering notes
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab-ce/issues/26723) in GitLab 11.5.