Removing pip install instruction for NeMo voice agent environment setting

[tango4j]

What does this PR do ?

Since NeMo voice agent is not pip install friendly, we are removing the pip-install instructions.

Collection: [Note which collection this PR will affect]

NeMo voice agent

Changelog

Remove the few lines in examples/voice_agent/README.md

Usage

See https://github.com/NVIDIA-NeMo/NeMo/tree/main/examples/voice_agent

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• [x] Make sure you read and followed Contributor guidelines
• [ ] Did you write any new necessary tests?
• [ ] Did you add or update any necessary documentation?
• [ ] Does the PR affect components that are optional to install? (Ex: Numba, Pynini, Apex etc)
  • [ ] Reviewer: Does the PR have correct import guards for all optional libraries?

PR Type:

• [ ] New Feature
• [x] Bugfix
• [x] Documentation

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Additional Information

• Related to # (issue)

[tango4j]

@chtruong814 This is document change PR so it can skip tests.