Remove duo_code_review_previous_discussions feature flag

What does this merge request do and why?

Duo Code Review now always passes previous review discussions to the flow, so it does not repeat earlier findings. The Code Review flow sets flow_version to ^2.0.0, like the other foundational flows.

This change needs an AI Gateway that has the code_review flow version 2.0.0 (GitLab 19.4 or later). On a GitLab Duo Self-Hosted instance with an older AI Gateway, the review fails. It does not fall back to version 1.0.0.

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