Remove /root/.ssh/authorized_key reference

Dokku-alt should not use the root users authorized_keys file as a reference. It might contain settings that prevents dokku to perform correctly and also could expose security issues.
I run into an issue with this after trying out dokku, which had an error in the authorized_keys file, but since I never logged in with the root user I was not aware of it.
It might be better to simply provide a default authorized_keys file.