If you want absolutely nothing to appear I recommend display:none; the difference is that with visibility:hidden; It is not visible but takes up its original space while display:none; It is hidden and does not take up space.
How to hide login & signup fields CSS
- Edited
oops social login not working, its stuck at signup page
we have to choose password na after social login , so we should not disable .Form-group {display:none;}
how to hide password field and login button ?
something like .Form-group.password {display:none;} ?
Could someone contribute css code please 
If I understand correctly you want to have only the buttons for social login. But you can not hide the form, because an input is necessary as soon as one uses the social login? I have not used this function until now. I'll have a look at it when I'm at home.
3 months later
- Edited
https://discuss.flarum.org/d/28424-third-party-login-only-block-standard-login
If you face any issues with this plugin after using for few months you may need to uninstall extension using SSH command, It's completely safe to use but just in case if you want to disable this if you get locked use the command to uninstall.
Hari it works with Steam Auth?