doc: remove AsyncWrap mentions from async_hooks.md
AsyncWrap is a private API and should not be mentioned in the documentation. For some reasons, it was there for ages.
cc @nodejs/async_hooks
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines