src: don't touch js object in Http2Session dtor

Don't try to update the internal field pointer of the JS object in the
destructor.

The garbage collector invokes the destructor when the object is
collected and is not necessarily in a valid state anymore.

Backport-PR-URL: https://github.com/nodejs/node/pull/19185
PR-URL: https://github.com/nodejs/node/pull/18656


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>