Remove the readme ignore_columns rule from bundled catalog Version

Follow-up to #624275 (closed).

Once the columns are dropped, remove the ignore_columns rule from Ci::Catalog::BundledResources::Version:

ignore_columns %i[readme readme_html], remove_with: '19.6', remove_after: '2026-10-15'

The three-line comment above it goes too.

Acceptance criteria

• The ignore rule and its comment are removed.