test: remove unnecessary assignments

common.js needs to be loaded in all tests so that there is checking
for variable leaks and possibly other things. However, it does not
need to be assigned to a variable if nothing in common.js is referred
to elsewhere in the test.

PR-URL: https://github.com/nodejs/node/pull/4408
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-console-instance.js

@@ -1,5 +1,5 @@
 'use strict';
-var common = require('../common');
+require('../common');
 var assert = require('assert');
 var Stream = require('stream');
 var Console = require('console').Console;