src: remove unused guards around node-api reference
This is roughly a revert on https://github.com/nodejs/node/pull/38250 while we have a formal fix around the use-after-free issue on v8impl::Reference
with https://github.com/nodejs/node/pull/38000 landed.
As https://github.com/nodejs/node/pull/38250 was alleviating the problem but the root cause is unfortunately not fixed, we may remove the code to prevent future confusion.