lib: remove broken NODE_MODULE_CONTEXTS feature

lib: remove broken NODE_MODULE_CONTEXTS feature

This feature has no tests and has been broken for ages, see for example
https://github.com/iojs/io.js/pull/1160.  Don't bother fixing it, it's
pretty much broken by design and there can't be too many users because
it's almost undocumented.  A quick Google search suggests that it causes
more grief than joy to the few that do use it.  Remove it.

PR-URL: https://github.com/iojs/io.js/pull/1162
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>