Fix Grounding DINO notebook widget metadata rendering

[rkrisa]

Summary

• Removes stale metadata.widgets from the Grounding DINO zero-shot object detection notebook.
• Keeps notebook cells and tutorial content unchanged.
• Addresses the GitHub rendering error reported in zero-shot-object-detection-with-grounding-dino.ipynb can't render correctly #438.

Validation

• Parsed the notebook as JSON.
• Validated the notebook with
  bformat==5.10.4.
• Confirmed metadata.widgets is absent and the notebook still contains 34 cells.

Fixes #438.

[review-notebook-app]

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