fix(files_reminders): Do not attempt to send reminders after a user has left a share

[Pytal]

Summary

In cases where a user is shared a file and leaves the share or the file becomes unshared, the notification remains and will attempt to be resent continually

This makes sure that the notification is properly deleted

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[Pytal]

/backport to stable31

[github-actions]

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