[NFC][libc++] Fix _LIBCPP_HAS_BITSCAN64 usage.

Seems line was accidentally left in
llvm-svn: 290924 86eebc5

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D97211

Author: mordante

libcxx/include/__bits

@@ -76,7 +76,6 @@ inline _LIBCPP_INLINE_VISIBILITY
 int __libcpp_ctz(unsigned long long __x) {
     unsigned long __where;
 #if defined(_LIBCPP_HAS_BITSCAN64)
-    (defined(_M_AMD64) || defined(__x86_64__))
   if (_BitScanForward64(&__where, __x))
     return static_cast<int>(__where);
 #else