remove duplicate LIBOQS_BRANCH option in config doc (#274) #258

Job	Run time
cygwin (windows-2019, win64, -DOQS_ALGS_ENABLED=STD)	4m 57s
msvc (windows-2019, win64, -DOQS_ALGS_ENABLED=STD, no-shared no-fips VC-WIN64A-masm)	2m 32s
msvc_native (windows-2019, win64, -DOQS_ALGS_ENABLED=STD, no-shared no-fips VC-WIN64A, .CMake/too...	4m 34s
msvc_native (windows-2019, win64, -DOQS_ALGS_ENABLED=STD, no-shared no-fips VC-WIN64A, .CMake/too...	3m 46s
msvc_native (windows-2022, win64, -DOQS_ALGS_ENABLED=STD, no-shared no-fips VC-WIN64A, .CMake/too...	4m 26s
msvc_native (windows-2022, win64, -DOQS_ALGS_ENABLED=STD, no-shared no-fips VC-WIN64A, .CMake/too...	4m 58s
	25m 13s

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