image-dired: Make HTML gallery generation obsolete

image-dired: Make HTML gallery generation obsolete

* lisp/image/image-dired.el (image-dired-gallery): Delete
defgroup again.
(image-dired-gallery-dir, image-dired-gallery-image-root-url)
(image-dired-gallery-thumb-image-root-url)
(image-dired-gallery-hidden-tags, image-dired-tag-file-list)
(image-dired-file-tag-list, image-dired--add-to-tag-file-lists)
(image-dired--add-to-file-comment-list)
(image-dired--create-gallery-lists, image-dired--hidden-p)
(image-dired-gallery-generate): Make obsolete.