Remove `XGBoosterGetModelRaw`. #9464

Job	Run time
Test installing XGBoost lib + building the C API demo (ubuntu-latest, 3.8)	6m 4s
Test Google C++ unittest (CPU Non-OMP) (ubuntu-latest)	8m 34s
Code linting for C++	49s
Test Google C++ test (CPU) (macos-11)	8m 52s
	24m 19s

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