Remove outdated AsyncWrap docs

[watson]

Since the AsyncHooks docs are now online, I can't think of a reason for the Diagnostics WG to maintain this documentation any more. As discussed in todays meeting I'm therefore making this PR to remove the old AsyncWrap docs from the repo.

The official docs can be found here:
https://nodejs.org/api/async_hooks.html

@AndreasMadsen: This PR is just a suggestion. Just let me know if you think we should do something else 😃

[AndreasMadsen]

Hehe, agreed. But we should update https://github.com/nodejs/diagnostics/blob/master/tracing/README.md too.

[watson]

@AndreasMadsen should I update (and rename) the AsyncWrap section in the tracing README.md or do you want me to just remove it completely?

I think it would be nice to have something still as it's a module that falls under our area. I can try and update this PR with a suggestion

[watson]

@AndreasMadsen For starters, I just made a minimal change to the tracing README.md now. I'm not sure if the actual text needs changing as well

[AndreasMadsen]

Maybe just:

AsyncHooks - Tracks the lifetime of asynconuse requests (init/destroy) and their callback activity (before/after).

We now track more than just the AsyncWrap C++ classes. In particular we allow node modules to use the Embedder API to make pools look correct (like setTimeout does).

[watson]

@AndreasMadsen fixed