chore(cleanup): remove the dead comments_moderation_mode column + handling
Native comments are gone; this column was set but never read. Removed from the site
INSERT (admin-sites create) and UPDATE (save) in lockstep (column + placeholder + value),
from ensurePrimarySite's seed INSERT, the ensureColumn in database.js, and the 3 unused
i18n keys (asite.comment_moderation / moderation_trust / moderation_moderate, x3 langs).
No DROP — the column stays in existing DBs as harmless dead data. require_login_to_comment
is kept (still used by PermissionsService).