Fix: Remove out dated demos

[gyliu513]

What does this PR do?

Remove out dated demos.

• DocQA: This is not working anymore and this RAG features already covered in agents/rag_agent.py
• ios_calendar_assistant, ios_quick_demo, android_app, mobile support for llamastack is not going to be maintained
• notebooks: Created two years ago, not working, there are some notebooks in llamastack main repo can replace those notebooks

Feature/Issue validation/testing/test plan

Please describe the tests that you ran to verify your changes and relevant result summary. Provide instructions so it can be reproduced.
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• Test A
  Logs for Test A

• Test B
  Logs for Test B

Sources

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Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

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