remove unused option

Why was this here?
image_list: split into several files
cache: split out into its own module

Comment it out from the commands until it is working again
image list manager: code cleanup
cache: enable cache again on its own module
image: do not use class attributes.
image list: add method to get list of subscribed images

Instead of making the comparison in other parts of the code it seems
reasonable to provide a method to get a list of the subscribed images,
if any.