remove DATAHUB_HOST environment variable

[edevosc2c]

Datahub is not part of the helm georchestra, so the host env shouldn't be included in the helm chart:
https://github.com/georchestra/helm-georchestra/blob/main/templates/_helpers-envs.tpl#L25C9-L25C21

On top of that, it's hardcoding the fact that the service datahub would be called "datahub-datahub-svc" but it could very well be another name. If the helm release is called "datahub2" then the service name will be called "datahub2-datahub-svc"

As this environment variable has probably already been used somewhere in the datadir, we should be careful of this removal.