After microsoft/GSL#821 "gsl_byte should inspect __cpp_lib_byte in addition to _HAS_STD_BYTE" is fixed, _HAS_STD_BYTE should no longer be needed for detection.

After the Windows SDK's unqualified mentions of byte (referring to their own ::byte) are fixed, _HAS_STD_BYTE should no longer be needed for control.

Then we can finally remove:

Also, I created microsoft/GSL#1145 so that GSL will no longer inspect _HAS_STD_BYTE at all.