The steps for removing the proprietary drivers are as follows:
$ sudo apt-get purge 'fglrx*'
$ sudo update-alternatives --remove-all x86_64-linux-gnu_gl_conf
$ sudo apt-get install --reinstall libgl1-mesa-dri libgl1-mesa-glx
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Ref: mwop.net/blog/2014-11-03-utopic-and-amd.html
