Remove onnx_backend_test.py from top level cmake #15782

Job	Run time
cancel	5s
licensing	10m 28s
linux (ubuntu-20.04, debug)	48m 34s
Check if image exists in registry	8s
linux (ubuntu-20.04, release)	38m 6s
linux (ubuntu-20.04, codecov)	38m 48s
pyflakes	5m 27s
linux-fpga (ubuntu-20.04, debug)	56m 9s
Build SLES image	0s
Build image	0s
sles	21m 9s
format	4m 43s
cppcheck	2m 21s
tidy	43m 39s
	4h 29m 37s

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