Race condition in deleting segments

[sentry]

Acceptance Criterion

• The asynchronous task delete_segment ignores it when the segment is already deleted.

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Sentry Issue: FLAGSMITH-API-5NB

Segment.DoesNotExist: Segment matching query does not exist.
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  File "segments/tasks.py", line 14, in delete_segment
    Segment.objects.get(pk=segment_id).delete()

Failed to execute task 'tasks.delete_segment', with id 410046727. Exception: Segment matching query does not exist.