Remove PBRT_CPU_GPU from various CPU-only functions #221

Job	Run time
Build and test g++-9 -	6m 15s
Build and test g++-10 -	5m 23s
Build and test clang++-10 -	5m 1s
Build and test clang++-11 -	5m 5s
Build and test g++-9 - -DPBRT_DBG_LOGGING=True	4m 5s
Build and test g++-10 - -DPBRT_DBG_LOGGING=True	4m 9s
Build and test clang++-10 - -DPBRT_DBG_LOGGING=True	4m 18s
Build and test clang++-11 - -DPBRT_DBG_LOGGING=True	3m 43s
Build and test g++-9 - -DPBRT_FLOAT_AS_DOUBLE=True	3m 57s
Build and test g++-10 - -DPBRT_FLOAT_AS_DOUBLE=True	3m 54s
Build and test clang++-10 - -DPBRT_FLOAT_AS_DOUBLE=True	4m 16s
Build and test clang++-11 - -DPBRT_FLOAT_AS_DOUBLE=True	4m 0s
	54m 6s

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