crypto: remove BIO_set_shutdown

I've not been able to find any reason for calling
BIO_set_shutdown(bio, 1). This is done by default for the following
versions of OpenSSL:

https://github.com/openssl/openssl/blob/OpenSSL_1_1_0/
crypto/bio/bio_lib.c#L26

https://github.com/openssl/openssl/blob/OpenSSL_1_0_1/
crypto/bio/bio_lib.c#L90

https://github.com/openssl/openssl/blob/OpenSSL_1_0_2/
crypto/bio/bio_lib.c#L88

https://github.com/openssl/openssl/blob/OpenSSL_1_0_0/
crypto/bio/bio_lib.c#L90

This commit removes the call and the comment.

Backport-PR-URL: #17784
PR-URL: #17542
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

src/node_crypto_bio.cc

@@ -51,8 +51,6 @@ void NodeBIO::AssignEnvironment(Environment* env) {
 int NodeBIO::New(BIO* bio) {
   bio->ptr = new NodeBIO();
 
-  // XXX Why am I doing it?!
-  bio->shutdown = 1;
   bio->init = 1;
   bio->num = -1;