src: remove redeclarations of variables

Remove redeclarations of variables in node.js. This includes removing
one apparently unnecessary `NativeModule.require('module')`.

PR-URL: https://github.com/nodejs/node/pull/4605


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: James M Snell <jasnell@gmail.com>