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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PR Type:

• New Feature
• Bugfix
• Documentation

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Additional Information

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[tango4j]

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