How do I remove the proprietary ATI drivers?

Try to completely remove your ATI drivers from your system:

sudo apt-get purge "fglrx.*"

Remove your xorg.conf

sudo rm /etc/X11/xorg.conf

Reinstall xorg completely

sudo apt-get install --reinstall xserver-xorg-core libgl1-mesa-glx:i386 libgl1-mesa-dri:i386 libgl1-mesa-glx:amd64 libgl1-mesa-dri:amd64

Re-configure Xorg

sudo dpkg-reconfigure xserver-xorg

Reboot

sudo reboot

You should be greeted with lightdm, this will default everything x the same way a fresh install would.

Remove the drivers, .deb or normal install (if you get a file not found ignore it)

sudo sh /usr/share/ati/fglrx-uninstall.sh
sudo apt-get remove --purge fglrx fglrx_* fglrx-amdcccle* fglrx-dev*

Remove your xorg.conf

sudo rm /etc/X11/xorg.conf

Reinstall xorg

sudo apt-get install --reinstall xserver-xorg-core libgl1-mesa-glx libgl1-mesa-dri libgl1-mesa-glx libgl1-mesa-dri

Configure Xorg

sudo dpkg-reconfigure xserver-xorg

Reboot:

sudo reboot

After the reboot all the fglrx packages will be gone, you will be using default open source.

For more information on how to remove / add / replace ATI drivers in your system there is already a very good post with these steps.

To remove all the current fglrx packages from your system

If any of these returns errors like file not found or package not found, ignore it.

Run these commands in a Terminal (Ctrl+Alt+T):

sudo sh /usr/share/ati/fglrx-uninstall.sh
sudo apt-get remove --purge fglrx fglrx_\* fglrx-amdcccle\* fglrx-dev\*