src: remove `HasWriteQueue()`

Tests are passing without it, and this otherwise makes the
code harder to reason about because the `async` flag on the
write request object would not be set even though the callback
would still be pending.

PR-URL: https://github.com/nodejs/node/pull/18019


Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>