Fix orphaned notification settings

What does this MR do?

Removes any NotificationSettings that remained after their parent namespace was deleted.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/29688

• Changelog entry added, if necessary
• 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