src: remove has_experimental_policy option

This would be set when `--experimental-policy` was set,
but since an empty string does not refer to a valid file,
we can just check the value of `--experimental-policy`
directly.

PR-URL: https://github.com/nodejs/node/pull/25628


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>