Remove DAST table from `known_cross_joins` list in desired_sharding_key_spec.rb

What does this MR do and why?

After transitioning all DAST tables to gitlab_sec schema they can be removed from known_cross_joins list

MR acceptance checklist

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Related issues:

• Fix cross-join between dast_scanner_profiles an... (#479866 - closed) • Schmil Monderer • 17.5
• Fix cross DB foreign key between dast_site_prof... (#477706 - closed) • Schmil Monderer • 17.5
• Fix cross-join in ee/spec/lib/gitlab/database/d... (#481764 - closed) • Schmil Monderer • 17.5