Remove redundant Python script formatting from README generation

[RobMulla]

Now that generate_support_matrices.sh natively injects fully-formed UI format strings directly into the upstream CSV deliverables, we no longer need downstream Python scripts attempting to intercept and reshape the statuses.

Changes

• Removed: python3 scripts/update_all_matrices.py from the Buildkite commit wrapper.
• Removed: python scripts/combine_matrices.py from the GitHub Action.
• Simplified: update_readme.py now points directly to the core model_support_matrix.csv.
• Refactored: merge_metrics() and generate_markdown_table() no longer blindly try to scrape emojis or reshape fallback values, relying exclusively on the upstream's single source of truth.

[github-actions]

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