http: remove reference to onParserExecute

Parsers hold a reference to the socket associated with the request
through onParserExecute. This must be removed when the parser is
freed so that the socket can be garbage collected when destroyed.

Regression introduced in commit 59b91f1 ("http_parser: consume
StreamBase instance").

PR-URL: https://github.com/nodejs/node/pull/4773


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>