benchmark: remove excessive value from http2 benchmark

`requests = 1000000` took about 10 minutes per run for me
and doesn’t seem to add much value on its own.

PR-URL: https://github.com/nodejs/node/pull/18936


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>