remove redundant function declaration: SameHemisphere() has been declared twice #206

Job	Run time
Build and test g++-9 -	8m 33s
Build and test g++-10 -	4m 49s
Build and test clang++-10 -	5m 3s
Build and test clang++-11 -	6m 50s
Build and test g++-9 - -DPBRT_DBG_LOGGING=True	3m 59s
Build and test g++-10 - -DPBRT_DBG_LOGGING=True	7m 45s
Build and test clang++-10 - -DPBRT_DBG_LOGGING=True	3m 57s
Build and test clang++-11 - -DPBRT_DBG_LOGGING=True	3m 50s
Build and test g++-9 - -DPBRT_FLOAT_AS_DOUBLE=True	4m 2s
Build and test g++-10 - -DPBRT_FLOAT_AS_DOUBLE=True	7m 10s
Build and test clang++-10 - -DPBRT_FLOAT_AS_DOUBLE=True	6m 15s
Build and test clang++-11 - -DPBRT_FLOAT_AS_DOUBLE=True	5m 44s
	1h 7m 57s

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