src: remove unused node_buffer.h include

I can't find any usage of Buffer in this file and think the include
can be removed.

PR-URL: https://github.com/nodejs/node/pull/13095
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

src/node_crypto_clienthello.cc

@@ -1,6 +1,5 @@
 #include "node_crypto_clienthello.h"
 #include "node_crypto_clienthello-inl.h"
-#include "node_buffer.h"  // Buffer
 
 namespace node {