Fix: When MR approvals are disabled, but approvers were previously assigned

What does this MR do?

Fix: When MR approvals are disabled, but approvers were previously assigned, all approvers receive a notification on every MR

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

dunno

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

closes https://gitlab.com/gitlab-org/gitlab-ee/issues/3104