COMP: Make Doxygen `IMAGE_PATH` empty
COMP: Make Doxygen `IMAGE_PATH` empty

Fixed Doxygen 1.9.1 warning:
> warning: source '<elastix-source>/dox/doxygen/images' is not a readable file or directory... skipping.

The doxygen `\image` command is not being used anyway, in the current elastix source tree.

SuperElastix/elastix,SuperElastix/elastix,SuperElastix/elastix,SuperElastix/elastix,SuperElastix/elastix