Remove ci_skip_fork_mr_lookup_for_non_forks feature flag

What does this MR do and why?

I'm removing the ci_skip_fork_mr_lookup_for_non_forks feature flag and keeping the enabled behaviour: Ci::Pipeline#all_merge_requests only runs the fork target-project pluck when the project is a fork.

The flag de-risked the fix for pipelines-page statement timeouts on projects with very large (source_project, source_branch) merge request buckets. It has been fully enabled on GitLab.com since 2026-08-12 (tracked in [FF] `ci_skip_fork_mr_lookup_for_non_forks` -- ... (#605142 - closed)) and logs have stayed healthy, so removal is the last step.

One behaviour note carried over from the introducing MR: projects unlinked from their fork network no longer get historical cross-project merge requests back from all_merge_requests. This was reviewed and accepted in this discussion on the introducing MR and stays covered by a spec.

References

• Rollout issue: [FF] `ci_skip_fork_mr_lookup_for_non_forks` -- ... (#605142 - closed)
• Flag introduced by: Skip fork MR lookup for non-forks in Ci::Pipeli... (!244120 - merged)
• Original report: https://gitlab.com/gitlab-com/request-for-help/-/work_items/4998+
• 100% enable on production: https://gitlab.com/gitlab-com/gl-infra/feature-flag-log/-/work_items/62278+

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