remove geometry-shader demos

It's been over a decade since mesa removed support for GL_ARB_geometry_shader4, meaning that this demo hasn't been working since then.

I don't think the value in keeping this test around is terribly high, as it's an very trivial test, that does almost nothing. And there's plenty of other sources of much more interesting geometry-shader content out there.

So let's just delete it. It has outlived it's usefulness.