tls_wrap: remove unused variable

Closes #7957.

lib/_tls_wrap.js

@@ -23,7 +23,6 @@
 // USE OR OTHER DEALINGS IN THE SOFTWARE.
 
 var assert = require('assert');
-var constants = require('constants');
 var crypto = require('crypto');
 var net = require('net');
 var tls = require('tls');