benchmark: removed unused arguments from callbacks

Removed unused arguments 'buf' and 'rinfo' from the callbacks.

PR-URL: https://github.com/nodejs/node/pull/14919


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>