GLX: Drop support for pre-Mesa-8.0 drivers
This started as an attempt to unify the GLX context creation logic to consistently go through the
create_context_attribs path internally. Support for that path was added for dri2 and drisw in 48ffc6a1 and 296fe21a in Mesa 8.0, which was February 2012 and supported nothing newer than Ivybridge, Turks and Caicos, and Fermi. Mesa 8.0 also happens to be the release in which we dropped the DRI1 drivers, so that seems like a nice clean break; if you're really trying to investigate a bug older than that, you can build your own libGL too.
In case that wasn't clear enough: this series drops support for loading DRI1 drivers, which last existed in Mesa 7.11.x.