Remove randomize_notification_email_marker feature flag

What does this MR do and why?

Remove the randomize_notification_email_marker feature flag and always use a random hidden footer marker in notification emails. This covers both the notify and mailer layouts, and prevents Gmail from collapsing notifications for the same target.

This MR follows Add random marker to mailer layout, which merged on 2026-10-02.

References

• Rollout: #630387 (closed)
• Bug: #630361 (closed)
• Introduced by: !257053 (merged)

Screenshots or screen recordings

Not applicable - the marker is hidden, and visible HTML is unchanged.

How to set up and validate locally

bundle exec rspec spec/helpers/emails_helper_spec.rb spec/mailers/emails/merge_requests_spec.rb spec/mailers/emails/profile_spec.rb

All three files pass locally at b8b8ebb4 (431 examples, 0 failures).

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist.

• Merge only after Add random marker to mailer layout is deployed and verified in Gmail. This MR also removes the flag check that MR added to mailer.html.haml.
• After merge and deployment, run the release check and feature delete commands in the rollout issue.