removed unused/useless requires in lib/dedupe.js

This PR removes the two duplicate lines `require('npm') ` on line 18 and 20 of dedupe.js. This is currently causing issues when installing the jshint package as a sub-dependency calls `npm dedupe` which raises the following error
````shell
npm ERR! Error: Cannot find module 'npm'
npm ERR!     at Function.Module._resolveFilename (module.js:338:15)
npm ERR!     at Function.Module._load (module.js:280:25)
npm ERR!     at Module.require (module.js:362:17)
Press ENTER or type command to continue
npm ERR!     at require (module.js:378:17)
npm ERR!     at Object.<anonymous> (/usr/lib/nodejs/npm/lib/dedupe.js:18:11)
npm ERR!     at Module._compile (module.js:449:26)
npm ERR!     at Object.Module._extensions..js (module.js:467:10)
npm ERR!     at Module.load (module.js:356:32)
npm ERR!     at Function.Module._load (module.js:312:12)
npm ERR!     at Module.require (module.js:362:17)
npm ERR! If you need help, you may report this log at:
npm ERR!     <http://github.com/isaacs/npm/issues>
npm ERR! or email it to:
npm ERR!     <npm-@googlegroups.com>
````