src: remove NODE_INVALID_UTF8 environment variable

Introduced in joyent/node v0.10 as a backwards compatibility measure.
It's an ugly hack and allowing invalid UTF-8 is not a good idea in the
first place, remove it.

PR-URL: https://github.com/iojs/io.js/pull/1042


Reviewed-By: Trevor Norris <trev.norris@gmail.com>