This option is based on a recommendation from a page last updated in
2014 (see https://www.freedesktop.org/wiki/Software/Glamor/), and it
is not necessary anymore.
Also, it did the wrong thing: it forced DRI2, but Glamor should also
work with DRI3, that is a better option most of the time. So let's
remove this option, folks that still want to force this manually can do
so in other ways.
