Remove unused ARC monitor path

[mzeng-openai]

Summary

• remove the unreachable ARC monitor path from MCP tool approval handling
• delete the unused ARC monitor module/tests and trim the orphaned safety-monitor decision plumbing
• keep always allow approvals on the existing auto-approval short-circuit without a dead monitor hop

Testing

• cargo test -p codex-core mcp_tool_call
• just fmt
• just fix -p codex-core
• git diff --check

Additional validation

• Attempted cargo test -p codex-core; the library test target passed, then the integration target failed in this local environment.
• The narrower MCP-focused rerun passed its unit coverage and only hit missing local test_stdio_server binaries in filtered integration cases.