image-dired: Minor cleanups

image-dired: Minor cleanups

* lisp/image/image-dired-dired.el
(image-dired-dired-after-readin-hook, image-dired-minor-mode):
Minor doc fixes.
(image-dired-dired-display-image): Don't accept ineffectual prefix
argument.
* lisp/image/image-dired-util.el
(image-dired-file-name-at-point): Use when-let.
(image-dired-display-thumb-properties): Redefine as obsolete function
alias for 'image-dired-update-header-line'.

* lisp/image/image-dired-util.el
(image-dired-window-width-pixels): Make obsolete in favor of
window-body-width.
* lisp/image/image-dired.el (image-dired-line-up-dynamic)
(image-dired-display-window-width): Don't use above obsolete function.