That's the easiest thing, then.
From the command prompt in recovery mode:
(Edit: from long familiarity, I forgot to tell you something obvious to me -- you have to mount the drive ...)
to mount the drive in read/write mode
mount -o remount,rw /
apt-get purge fglrx* fglrx-amdcccle*
The first command removes the driver, the second renames the xorg.conf file. You can remove it later, but I generally recommend renaming things initially just in case you decide you want them back.
mv /etc/X11/xorg.conf /etc/X11/xorg.conf.bak