Remove use_namespace_id_for_issue_and_work_item_finders stub

What does this MR do and why?

This MR removes the global use_namespace_id_for_issue_and_work_item_finders stub from spec/spec_helper.rb. That stub was added in !207030 (merged) to work around failures in group issues list specs when the flag was enabled.

The root cause was in IssuableFinder#by_non_archived, which called Issue.non_archived(use_existing_join: use_join_strategy_for_project?) and told the scope to skip its own projects join in favour of a join that did not actually exist in that code path. That branch was removed in !217011 (merged). The finder now always calls plain items.non_archived, and the scope builds its own left_joins(:project) independent of the finder's join strategy.

With that fix in place, specs pass with the flag enabled, confirmed by a green full pipelinetier-3 on this MR, so the stub is no longer needed.

Since specs enable all feature flags by default, removing the stub means specs now exercise the flag-enabled code path going forward.

References

• !207030 (merged)
• !217011 (merged)

Screenshots or screen recordings

Not provided

How to set up and validate locally

• Run a previously affected spec file on this branch, for example bin/rspec spec/finders/work_items_finder_spec.rb. Since specs enable all feature flags by default, this now exercises the use_namespace_id_for_issue_and_work_item_finders enabled path and passes

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