test: remove Object.observe from tests

Testing this wasn't really useful, besides Object.observe is going to be
deprecated.

Also this test fails with Chakra (#4765) for obvious reason.

PR-URL: https://github.com/nodejs/node/pull/4769


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>