src: don't call into VM from AsyncWrap destructor
R=@trevnorris?
See #8216 (closed) and #9465 (closed), it's making node crash.
It might be possible to retain the destroy hook by maintaining a per-environment list + a uv_idle_t handle or something like that but it's a lot of work and there might be edge cases so I'm opting for simply removing the hook. Whoever disagrees volunteers to do the hard work. :-)