Remove FinishedBuildsDeduplicatedFinder and deprecated mode argument

What does this MR do and why?

Follow-up cleanup after the BackfillCiFinishedBuildsToClickHouse BBM and mode-split removal. Two items were deliberately deferred one release to respect the Sidekiq backward-compatibility guidelines and keep the previous change focused.

Closes #604160.

Stacked MR: This targets the Plan A branch narendran-kannan/592894/remove-backfill-bbm-modesplit (!247908 (merged)) because it depends on that MR having removed the mode: parameter from the sync service and the backfill_in_progress? branch that was the finder's only production caller. Retarget to master once !247908 (merged) merges.

What this MR removes

• ClickHouse::Finders::Ci::FinishedBuildsDeduplicatedFinder and its spec. It was only used by the backfill_in_progress? branch in Ci::JobAnalytics::QueryBuilder, which was removed in !247908 (merged). It is now unused in production code. Two comments in the siphon finders that referenced it as a conceptual analogue are updated accordingly.
• The deprecated _mode = nil argument from ClickHouse::CiFinishedBuildsSyncWorker#perform, plus the now-obsolete loggable_arguments index. It was kept for one release per the Sidekiq compatibility guidelines so in-flight jobs scheduled with the old 3-arg signature would still execute. By 19.3 no such jobs can remain, so the argument is dropped.

Verification

$ bundle exec rspec \
    ee/spec/workers/click_house/ci_finished_builds_sync_worker_spec.rb \
    spec/lib/ci/job_analytics/query_builder_spec.rb
...
62 examples, 0 failures

RuboCop is clean on the touched files. Confirmed via grep that there are no remaining references to the deleted finder and no caller passes a third argument to the sync worker.

MR acceptance checklist

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