test: remove potential race condition in https renegotiation test

In test/pummel/test-https-ci-reneg-attack.js, there is a boolean that is
causing a race condition on some operating systems. It is unnecessary.
Remove it.

PR-URL: https://github.com/nodejs/node/pull/25601


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>