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/stdlib.h b/include/stdlib.h
index 64581b6..e4dce9c 100644
--- a/include/stdlib.h
+++ b/include/stdlib.h
@@ -90,9 +90,7 @@
 #  pragma GCC system_header
 #endif
 
-#  if __has_include_next(<stdlib.h>)
-#    include_next <stdlib.h>
-#  endif
+#include_next <stdlib.h>
 
 #ifdef __cplusplus
 extern "C++" {