CLN: remove duplicate functions in two SPH backend PRs

[nastasha-w]

TODO

After merging two PRs with fixes for the SPH projection and Ray backends (#4939 and #4783), import some functions (_cubicspline_python and _integrate_kernel) for the Ray tests from yt/testing.py (as used in yt/geometry/coordinates/tests/test_sph_pixelization.py in the sph_proj_backend branch) instead of defining them in their own test script (yt/data_objects/tests/test_rays.py in the issue4781_sph_ray_dl branch). This issue is a reminder to clean those duplicate definitions up after both PRs are merged.

[nastasha-w]

Since #4939 has been merged into main, I merged the newest main version into the #4783 branch, and removed the duplicate functions there in a new commit.
I think we can close this issue after that PR is merged.