Cleanup undefined severity level from code

What does this MR do?

Recently we deprecated undefined severity level for. This is cleanup from the code. undefined confidence is still exists.

!25884 (merged) -> we no longer accept undefined severity

For existing values in database we ran background migration

SELECT Count(*) FROM   vulnerability_occurrences WHERE  severity = 0

Aggregate  (cost=4.14..4.15 rows=1 width=8) (actual time=0.934..0.934 rows=1 loops=1)
   Buffers: shared read=1
   I/O Timings: read=0.921
   ->  Index Only Scan using undefined_vulnerabilities on public.vulnerability_occurrences  (cost=0.12..4.14 rows=1 width=0) (actual time=0.929..0.929 rows=0 loops=1)
         Heap Fetches: 0
         Buffers: shared read=1
         I/O Timings: read=0.921

Related issue

#198158 (closed)

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