Remove Okta auth provider for self-hosters

We support allowing configuring Okta as a socal provider via environment variable configuration (`AUTH_OKTA*`).

However, compared to the SSO configuration that can be done on the organization level it lacks a lot of features. Therefore, we recommend self-hosters to instead create an organization and set up SSO/OIDC for that organization. To avoid confusion and reducing maintenance burden we can remove the configuration via the `AUTH_OKTA*` environment variables.