lib: remove unused modules

Some files in `lib` were using `require` to load modules that were
subsequently not used in the file. This removes those `require`
statements.

PR-URL: https://github.com/nodejs/node/pull/4396


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>