docs: Drop node.gif

docs: Drop node.gif

It's the only GIF file we have in our repository.

We could convert it to PNG, but that would result in a
significantly bigger file (~5.5 KiB vs ~1.5 KiB).

Since the image doesn't really add much to the documentation,
drop it instead.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>