Remove onnx_backend_test.py from top level cmake

[attila-dusnoki-htec]

Currently when anything is changed in onnx_backend_test.py, a brand new build will be triggered.
This test is added correctly here to cmake.
This seems like an old artifact, or maybe i missed something. If so, please close this PR.

[codecov]

Codecov Report

Merging #2256 (2969a8b) into develop (4188c38) will increase coverage by 0.00%.
Report is 5 commits behind head on develop.
The diff coverage is 100.00%.

❗ Current head 2969a8b differs from pull request most recent head f558d21. Consider uploading reports for the commit f558d21 to get more accurate results

@@           Coverage Diff            @@
##           develop    #2256   +/-   ##
========================================
  Coverage    91.49%   91.50%           
========================================
  Files          430      430           
  Lines        16129    16133    +4     
========================================
+ Hits         14758    14762    +4     
  Misses        1371     1371           

Files	Coverage Δ
src/onnx/parse_constant_of_shape.cpp	92.85% <100.00%> (+1.19%)	⬆️