Clean up keymap and menu code in image-dired

Clean up keymap and menu code in image-dired

A function to populate these maps is not necessary, just define the maps
once and for all.
* lisp/image-dired.el (image-dired-show-all-from-dir): Make prompt
clearer and in the spirit of dired's.
(image-dired-define-display-image-mode-keymap):
(image-dired-define-thumbnail-mode-keymap): Remove.
(image-dired-thumbnail-mode-map):
(image-dired-thumbnail-mode-line-up-map):
(image-dired-thumbnail-mode-tag-map): Assimilate all define-key and mode
menu code from the aforementioned removed functions.  Reorder so that
the definitions are inherited properly.
(image-dired-display-current-image-sized): Fix erroneous message.
(image-dired-thumbnail-mode):
(image-dired-display-image-mode): Remove defunct call.