http2: simplify timeout tracking

There’s no need to reset the chunk counter for every write.

PR-URL: https://github.com/nodejs/node/pull/19206


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>