Fix COCO labels: google-coral model uses 0-indexed class IDs

The model from google-coral/test_data starts class IDs at 0 (person),
not 1 (background). Remove the placeholder that was causing all labels
to be shifted by one position.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/models/coco_labels.txt

@@ -1,4 +1,3 @@
-???
 person
 bicycle
 car