test: remove unused var in net-server-try-ports

`connections` is assigned but never used. Remove it.

(This was missed by the linter in previous versions of ESLint but is
flagged by the current version. Updating the linter is contingent on
this change or some similar remedy landing.)

PR-URL: https://github.com/nodejs/node/pull/7597
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>

test/parallel/test-net-server-try-ports.js

@@ -5,19 +5,15 @@ require('../common');
 var assert = require('assert');
 var net = require('net');
 
-var connections = 0;
-
 var server1listening = false;
 var server2listening = false;
 var server2eaddrinuse = false;
 
 var server1 = net.Server(function(socket) {
-  connections++;
   socket.destroy();
 });
 
 var server2 = net.Server(function(socket) {
-  connections++;
   socket.destroy();
 });