crypto: remove unused header in clienthello.h

This commit removes stdlib.h header as it does not seem to be used any
more.

PR-URL: #17752
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

Author: danbev

src/node_crypto_clienthello.h

@@ -5,7 +5,6 @@
 
 #include <stddef.h>  // size_t
 #include <stdint.h>
-#include <stdlib.h>  // nullptr
 
 namespace node {
 namespace crypto {