Remove tls1_alert_code().

Remove tls1_alert_code().

After running the preprocessor, this function becomes:

  switch (code) {
  case 0:
   return (0);
  case 10:
   return (10);
  case 20:
   return (20);
  ...
  }

Its intended purpose was to prevent SSLv3 alerts being sent from TLS code,
however now that we've removed "no_certificate" from LibreSSL's reach, it
no longer does anything useful.

ok tb@