Remove `namespace_id` from `vulnerability_reads` table

What does this MR do and why?

This MR removes namespace_id references from vulnerability_reads database triggers and then drops the column.

All other usages of this column were removed in %17.4 with !163087 (merged), but the removal of this column had to be pushed back one milestone as it was dependent on Finalize `ResolveVulnerabilitiesForRemovedAnaly... (!169339 - merged).

Relates to: #480196 (closed)

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