test: fix flaky test-http2-session-unref

This test should exit naturally or will timeout on its own,
a separate unrefed timer is not necessary.

PR-URL: https://github.com/nodejs/node/pull/18589
Fixes: https://github.com/nodejs/node/issues/18587


Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>