Remove dead external log sink settings (elasticsearch, syslog, webhook)

[pratyushsinghal7]

Pull Request

🔗 Related Issue

Part of #5848 (the three logging-sink flags; the remaining five flags need wiring up and are left for follow-ups)

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📝 Summary

Removes the dead "External Log Integration" settings block from mcpgateway/config.py — elasticsearch_enabled, syslog_enabled, webhook_logging_enabled and their companion fields (elasticsearch_url, elasticsearch_index_prefix, syslog_host, syslog_port, webhook_logging_urls). No code reads any of them, so setting e.g. ELASTICSEARCH_ENABLED=true silently does nothing. Per #5848, removal is the right resolution for these three sinks (they look like never-implemented companions to structured_logging_external_enabled).

Also removes the corresponding entries from the Helm chart (values.yaml, values.schema.json, chart README). They were not present in .env.example. docs/docs/design/images/classes.svg still mentions them but is a generated diagram.

.secrets.baseline was regenerated by the pre-commit hook (line-number shifts only).

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📏 Reviewability

• This PR has one clear purpose
• The linked issue is not labeled triage
• Unrelated bugs or improvements are tracked in separate issues/PRs
• Tests are included with the code they validate (no tests referenced these settings; removal only)
• If AI-assisted, I understand and can explain the generated changes

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🏷️ Type of Change

• Bug fix
• Feature / Enhancement
• Documentation
• Refactor
• Chore (deps, CI, tooling)
• Other (describe below)

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🧪 Verification

Check	Command	Status
Ruff	ruff check mcpgateway/config.py	✅ pass
YAML lint	yamllint -c .yamllint charts/mcp-stack/values.yaml	✅ pass
Pre-commit hooks	pre-commit run (on changed files)	✅ pass
Dead-flag check	grep -rn for each removed name (snake_case and UPPER_CASE)	✅ no hits outside generated classes.svg

Full make test was not run locally (Windows dev box); relying on CI.

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✅ Checklist

• Code formatted (make black isort pre-commit)
• Tests added/updated for changes (n/a — settings removal, no tests referenced them)
• Documentation updated (if applicable)
• No secrets or credentials committed