cleanup mbstowcs wrapper

remove unneeded headers. this file is utterly trivial now and there's
no sense in having a comment to state that it's in the public domain.

src/multibyte/mbstowcs.c

@@ -1,15 +1,5 @@
-/* 
- * This code was written by Rich Felker in 2010; no copyright is claimed.
- * This code is in the public domain. Attribution is appreciated but
- * unnecessary.
- */
-
 #include <stdlib.h>
-#include <inttypes.h>
 #include <wchar.h>
-#include <errno.h>
-
-#include "internal.h"
 
 size_t mbstowcs(wchar_t *restrict ws, const char *restrict s, size_t wn)
 {