Delete YOLO backend for ObjectDetector

[bomanimc]

We would like to remove YOLO support from the Object Detector class, which aligns with our general deprecation of the YOLO model integration from ml5.js.

From @joeyklee in #1183:

I think we can remove YOLO from the Object Detector entirely. The way I kind of see it right now, is that we are starting to land on these kind of "high level" model support -- e.g. object detector, face tracking, hand tracking, pose tracking -- and we have some pre-trained model that allows them to run. In ttrying to forefront maintainability, I think it doesn't hurt us to prune things out while we find our bearings amidst such quick growth and all the world happenings, and then work to add things back or build on top of as capacity permits. Does that resonate at all?

From @shiffman in #1183:

I'm good with this change, I may have referenced YOLO in some tutorials but I can update video descriptions / code examples to use COCOSSD only.