build: remove tools/wrk from the tree

wrk is an optional tool that some of the http benchmarks uses. The removal
doesn't affect any users. Developers are assumed to install it before running
the tests.

This change reduces the tarball by 5%

PR-URL: https://github.com/iojs/io.js/pull/982


Reviewed-By: Rod Vagg <rod@vagg.org>