Remove testing ort-nightly for Mac Python 3.6 due to unsupported ort-nightly (#3953)
Remove testing ort-nightly for Mac Python 3.6 due to unsupported ort-nightly (#3953)

* Remove testing ort-nightly for Mac Python 3.6 due to unsupported packages
onnx/onnx,onnx/onnx,onnx/onnx,onnx/onnx