Fix QNN CI job (#9774)

Summary:

1. setup-linux.sh is calling install_executorch.sh already so let's
remove the redundant call.
2. Migrate the legacy `executorch.extension.llm.tokenizer` code in
`llama.py` to `pytorch_tokenizers`.

Test Plan:

Reviewers:

Subscribers:

Tasks:

Tags:

### Summary
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### Test plan
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