Cleanup Detector labelmap

[NateMeyer]

1. Fixup OpenVino labelmap to use Car instead of Truck
2. Add missing indexes to default labelmap.txt
3. Remove TensorRT-specific labelmap, use default.
4. Fill any empty label indexes with "unknown"

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[NateMeyer]

The only reason OpenVino still has a unique labelmap is because the indexes are shifted one (index zero is "background"). Alternatively, we could decrement the detected label indexes in the OpenVino detector or make the index decrement a model parameter. That could get us to a common labelmap across all the detectors.

[blakeblackshear]

It's a small file and I don't expect to have all models share a common labelmap anyway.