XML export --half fix

[glenn-jocher]

Improved error reporting for #8519

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Enhancement of the model conversion compatibility in yolov5 export process.

📊 Key Changes

• 🛠 Improved assertion logic in the export script to relax constraints on using --half precision.
• 🗑️ Removed the unnecessary check for OpenVINO's XML format when using --half precision.

🎯 Purpose & Impact

• 🎨 The update allows users more flexibility by enabling half-precision model export (--half) on CPU for CoreML models, which wasn't allowed before.
• ✨ It removes a constraint that could cause confusion or errors when exporting models for different platforms.
• 👥 Users working with CoreML will benefit from performance improvements due to half-precision exports without needing a GPU.