tls: remove tls.createSecurePair code deprecation

In https://github.com/joyent/node/pulls/8695, the deprecation was
removed from doc.  This removes the deprecation from the code.

PR-URL: #8700
Reviewed-by: Chris Dickinson <christopher.s.dickinson@gmail.com>

lib/tls.js

@@ -246,8 +246,4 @@ exports.TLSSocket = require('_tls_wrap').TLSSocket;
 exports.Server = require('_tls_wrap').Server;
 exports.createServer = require('_tls_wrap').createServer;
 exports.connect = require('_tls_wrap').connect;
-
-// Legacy API
-exports.__defineGetter__('createSecurePair', util.deprecate(function() {
-  return require('_tls_legacy').createSecurePair;
-}, 'createSecurePair() is deprecated, use TLSSocket instead'));
+exports.createSecurePair = require('_tls_legacy').createSecurePair;