Discontinue ams-ps and avoid unnecessary reverse proxying

[hellais]

While updating the tor targets we ran into a situation where the configuration of backend-fsn was reverse proxying requests over to orchestrate.ooni.io (ie. ams-ps).

This lead to the surprise that updates to the target file inside of backend-fsn was not having any effect. We should make sure these targets are in fact retrieved from the backend-fsn host and not orchestrate.

This is the relevant configuration section: https://github.com/ooni/sysadmin/blob/master/ansible/roles/ooni-backend/templates/nginx-api-fsn.conf#L114.

According to #746, we only really need orchestrate still for register and login, but we could probably port those over so we can entirely discontinue this host.

At bare minimum we should adjust the nginx config such that we are only reverse proxying those two paths and not the whole /api/v1/test-list/* prefix.