chore(BA-941): clean up unused configuration of webserver.conf

[ironAiken2]

resolves #nnn (BA-941)

Related PR : lablup/backend.ai-webui#3325

Removes unused configuration options from the web server:

• Removes brand field from UI section
• Removes enable_model_store from service section
• Removes enable_LLM_playground from UI section

Checklist:

• Milestone metadata specifying the target backport version
• Mention to the original issue
• Installer updates including:
  • Fixtures for db schema changes
  • New mandatory config options
• Update of end-to-end CLI integration tests in ai.backend.test
• API server-client counterparts (e.g., manager API -> client SDK)
• Test case(s) to:
  • Demonstrate the difference of before/after
  • Demonstrate the flow of abstract/conceptual models with a concrete implementation
• Documentation
  • Contents in the docs directory
  • docstrings in public interfaces and type annotations

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📚 Documentation preview 📚: https://sorna--3934.org.readthedocs.build/en/3934/

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📚 Documentation preview 📚: https://sorna-ko--3934.org.readthedocs.build/ko/3934/

[ironAiken2]

• chore(BA-941): clean up unused configuration of webserver.conf #3934 👈 (View in Graphite)
• main

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[agatha197]

LGTM