[bugfix,documentation] Ship the image files with the gg_ prefix

These are the ones actually used in the doxygen documentation.
gg_ stands for 'genericgeometry_'.  The ones without the prefix show
the old numbering used before the advent of the generic implementation.
[cleanup,documentation] Remove unused drawings of the reference elements

This drawings show the numbering of the reference elements from the
pre-generic-referenceelements-era.  They are obsolete, and not referenced
anywhere in the module.  This patch removes them.