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: #18589
Fixes: #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>

test/parallel/test-http2-session-unref.js

@@ -49,5 +49,3 @@ server.listen(0, common.mustCall(() => {
 }));
 server.emit('connection', serverSide);
 server.unref();
-
-setTimeout(common.mustNotCall(() => {}), 1000).unref();