Remove OpenVINO ONNX opset<=12 check

[glenn-jocher]

No longer needed.

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Simplification of export conditions in export.py.

📊 Key Changes

• Removed the line of code that forces the OpenVINO model export to use ONNX opset version 12.

🎯 Purpose & Impact

• The goal is to streamline the export process by not explicitly limiting the OpenVINO export to a specific ONNX opset.
• Impact to users includes potentially greater flexibility and compatibility with various versions of OpenVINO without being restricted to opset 12.
• Developers may see a simplified codebase, making maintenance and further development easier.
• Users may need to ensure their OpenVINO setup is compatible with the opset version used by YOLOv5 for successful model deployment.