glsl: remove long-time unsupported demos
These demos depends on ARB_geometry_shader4 support, which was removed from Mesa back in 2015, with no plans to add it back. This means these demos can't be run on modern open-source drivers any more, so there's no point in carrying them around in the tree either.
In addition, porting these to newer GL versions is a lot of work, and will most likely be more like a complete rewrite. Also, it doesn't seem like anyone is planning on putting in the work. If someone steps up, they can dig out the sources from the git history.