You could use
sudo delgroup vboxusers to completely remove that group, if the uninstallation of Virtualbox failed to do that. Then enter
id in the terminal and your uid, gid, and the groups you belong to will be listed, and
vboxusers shouldn't be there.
If this does not work you could simply reinstall Virtualbox with
sudo apt-get install --reinstall <virtualbox pkg names> and then you might be able to remove the package cleanly with
sudo apt-get purge <virtualbox version> (which would remove any configuration files) and then you could use
sudo apt-get autoremove to remove any other files left by Virtualbox. Then you could try my first suggestion again.
Lastly, if these options are unsuccessful, but installing Virtualbox does show that group 125 is virtualbox, you could remove the Virtualbox users group (as detailed in my first suggestion with
sudo delgroup vboxusers), before you uninstall Virtualbox. This may work as Virtualbox doesn't seem to be uninstalling correctly and seems to be leaving a partial group that can't easily be removed after uninstallation as it doesn't show up in /etc/group.
It also should be said that you need to logout and login again for any changes to users and groups to take effect.