Clean up dangling references to removed regression scripts #3252

Summary

PR #3251 removes obsolete Python regression scripts, but review comments identified remaining references that should be cleaned up separately rather than blocking that PR.

Dangling references from PR review

• docs/experiments-beir-fusion.md:203-205 still documents python src/main/python/run_fusion_regression.py.
• src/main/python/beir/generate_template.py:68 still emits python src/main/python/run_regression.py --index --verify --search --regression ${test_name}.
• src/main/python/bright/generate_doc_templates.py:46 still emits the same removed run_regression.py entrypoint.

Additional follow-up

A local search also finds many generated reproduction docs under docs/reproduce/from-document-collection/ that mention run_regression.py. These should be reviewed and migrated to the current Java reproduction drivers where appropriate:

• io.anserini.reproduce.ReproduceFromPrebuiltIndexes
• io.anserini.reproduce.ReproduceFromDocumentCollection

Context

This issue tracks the dangling documentation/template references separately from #3251 so the cleanup can be handled in a focused follow-up change.