fix: remove the unused retry index field

[Nov1c444]

Summary

The retry_index field in the node_executions table was not actually functioning, so it has been removed. Additionally, the issue with retry logs in iteration nodes has been fixed.

Fixes #11871
Fixes #11896

Checklist

Important

Please review the checklist below before submitting your pull request.

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods