benchmark: clean up config resolution in multiple benchmarks

This removes 'to Number' casting in multiple benchmarks (which is
handled by the benchmark runner) and cleans up some var usage in changed
benchmarks.

PR-URL: https://github.com/nodejs/node/pull/31581


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rich Trott <rtrott@gmail.com>