benchmark: refactor _http-benchmarkers.js

Refactor _http-benchmarkers.js:

* The file used a mixture of inline callbacks with the `function`
  keyword and arrow functions. Use arrow functions for consistency.
* The file used a mixture of trailing commas and no trailing commas. Use
  trailing commas for consistency.

PR-URL: https://github.com/nodejs/node/pull/25803


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>