[libc++][NFC] Remove unused macro definition

Author: philnik777

libcxx/include/__config

@@ -1389,9 +1389,6 @@ __sanitizer_verify_double_ended_contiguous_container(const void*, const void*, c
 #    define _LIBCPP_WORKAROUND_OBJCXX_COMPILER_INTRINSICS
 #  endif
 
-// TODO: Make this a proper configuration option
-#  define _PSTL_PAR_BACKEND_SERIAL
-
 #  define _PSTL_PRAGMA(x) _Pragma(#x)
 
 // Enable SIMD for compilers that support OpenMP 4.0