remove nginx_stage User#groups calculation

[johrstrom]

We had this issue coming from discourse - https://discourse.openondemand.org/t/oidc-sssd-error-on-login/3926/8

where I'm finding that we don't use groups of the user anymore.

I tracked the addition to this commit - 3e94404

I am however unable to find the commit that removed the validate_user_group hook, so it's not clear why they removed this feature without also removing the group calculation.