[CUDA] remove src/treelearner/kernels #8773

Job	Run time
regular (macos-13, Python 3.10)	20m 35s
sdist (macos-13, Python 3.11)	12m 0s
bdist (macos-13, Python 3.8)	12m 36s
Python - latest versions (ubuntu-latest)	4m 25s
if-else (macos-13, Python 3.9)	4m 6s
bdist wheel (macos-14, Python 3.10)	4m 0s
Python - oldest supported versions (ubuntu-latest)	3m 18s
mpi source (macos-13, Python 3.11)	14m 27s
mpi pip (macos-13, Python 3.12)	13m 16s
mpi wheel (macos-13, Python 3.9)	17m 36s
all-python-package-jobs-successful	1s
	1h 46m 20s

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