Remove references to non-existent images

I ran into some problems deploying TimelineJS on a project which uses Django's staticfiles for deployment because staticfiles expects the image files referenced from CSS to exist. The current v2.33.1 build includes references to FancyBox's ``overlay.png`` and a non-existent ``loading@2x.gif``, neither of which exist.

I removed FancyBox outright because nothing else references it.

I changed the ``loading@2x.gif`` references to ``loading.gif`` but left the classes intact in case someone later adds a high-res version.