contextify: cleanup weak ref for global proxy

Cleanup how node_contextify keeps weak references in order to prepare
for new style phantom weakness API. We didn't need to keep a weak
reference to the context's global proxy, as the context holds it.

PR-URL: https://github.com/nodejs/node/pull/5392


Reviewed-By: Reviewed-By: bnoordhuis - Ben Noordhuis <info@bnoordhuis.nl>