Revert "[libc++] Only include_next C library headers when they exist"

This reverts commit 226409c62879bf5ff9928cd23a4255cd7c614fe0.
Breaks check-clang on mac, see comments on https://reviews.llvm.org/D136683

NOKEYCHECK=True
GitOrigin-RevId: 674729813e3be02af7bda02cfe577398763f58b9
diff --git a/include/stdint.h b/include/stdint.h
index fa7b011..ee71f62 100644
--- a/include/stdint.h
+++ b/include/stdint.h
@@ -120,8 +120,6 @@
 #   define __STDC_CONSTANT_MACROS
 #endif
 
-#if __has_include_next(<stdint.h>)
-#  include_next <stdint.h>
-#endif
+#include_next <stdint.h>
 
 #endif // _LIBCPP_STDINT_H