Don't show an "Unsubscribe" link in snippet comment notifications

What does this MR do?

Don't show an "Unsubscribe" link in snippet comment notifications.

Are there points in the code the reviewer needs to double check?

Screenshots (if relevant)

Before	After

Also, visiting an unsubscribe link (present in a notification sent before this change of course) for a snippet will now result in a 404:

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #23888 (closed)