Follow-up from "Resolve "Remove ghost notification settings for groups and projects""

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The following discussion from gitlab-ce!20567 should be addressed:

We need to remove notification_settings with orphan members.

In order to do that we could use a background migration: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20567#note_90488756

Edited by 🤖 GitLab Bot 🤖