[libc++] Remove a few __has_foo defines in __config

All the compilers we support implement those macros or builtins,
so it's not useful to have a fallback to 0 when they're not implemented.

Author: ldionne

libcxx/include/__config

@@ -334,26 +334,6 @@ _LIBCPP_HARDENING_MODE_DEBUG
 #    define _LIBCPP_CXX03_LANG
 #  endif
 
-#  ifndef __has_attribute
-#    define __has_attribute(__x) 0
-#  endif
-
-#  ifndef __has_builtin
-#    define __has_builtin(__x) 0
-#  endif
-
-#  ifndef __has_extension
-#    define __has_extension(__x) 0
-#  endif
-
-#  ifndef __has_feature
-#    define __has_feature(__x) 0
-#  endif
-
-#  ifndef __has_cpp_attribute
-#    define __has_cpp_attribute(__x) 0
-#  endif
-
 #  ifndef __has_constexpr_builtin
 #    define __has_constexpr_builtin(x) 0
 #  endif
@@ -375,10 +355,6 @@ _LIBCPP_HARDENING_MODE_DEBUG
 
 #  define __has_keyword(__x) !(__is_identifier(__x))
 
-#  ifndef __has_include
-#    define __has_include(...) 0
-#  endif
-
 #  ifndef __has_warning
 #    define __has_warning(...) 0
 #  endif