Remove remnants of old reftest system

Back in 2012, Brian wrote a reftest system using Firefox via Selenium to
generate comparison images.  This seems like a pretty cool idea, but we
didn't maintain it and important pieces are now missing.

Let's remove it from the working tree to avoid confusion with the current
reftest system, and I'll open a ticket about revisiting this idea.

One pair of old reftest files seems usable as a new reftest and that move is
included in this commit.