I have removed VirtualBox but I forgot to remove VirtualBox extensions & guest additions

The guest additions are inside your VMs, not on the VM host.

For the extensions, check if you still have a directory /usr/lib/virtualbox.

For a more graphical approach, try this:

https://github.com/shundhammer/qdirstat/blob/master/doc/Unpkg-View.md https://github.com/shundhammer/qdirstat/blob/master/doc/Pkg-View.md

with

qdirstat pkg:/virtual

and

xhost +
sudo qdirstat unpkg:/

I believe the guest extensions would be removed with the uninstall of VirtualBox VM's. So you can remove those if you aren't going to use them anymore, or reinstall VirtualBox and delete the VM's from there then do the uninstall again.