test: remove duplicate fork module import

`fork` is imported twice in a row. Remove duplication.

PR-URL: #4634
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>

test/parallel/test-child-process-fork-close.js

@@ -1,7 +1,6 @@
 'use strict';
 var assert = require('assert'),
     common = require('../common'),
-    fork = require('child_process').fork,
     fork = require('child_process').fork;
 
 var cp = fork(common.fixturesDir + '/child-process-message-and-exit.js');