Remove all remaining uses of gnus-group-decoded-name

Remove all remaining uses of gnus-group-decoded-name

* lisp/gnus/gnus-art.el (gnus-article-setup-buffer):
* lisp/gnus/nnrss.el (nnrss-retrieve-groups):
* lisp/gnus/message.el (message-forward-subject-author-subject):
  (message-forward-subject-name-subject):
* lisp/gnus/gnus-msg.el (gnus-inews-insert-gcc):
  (gnus-inews-make-draft):
* lisp/gnus/gnus-group.el (gnus-group-compact-group):
  (gnus-group-list-active):
  (gnus-group-kill-group):
  (gnus-group-set-current-level):
  (gnus-group-expire-articles-1):
  (gnus-group-catchup-current):
  (gnus-group-edit-group):
  (gnus-group-rename-group):
  (gnus-group-delete-group):
  (gnus-group-name-at-point): Remove calls in all these places, group
  names are always decoded.
* lisp/gnus/gnus-cache.el: Remove variables
  gnus-cache-unified-group-names and gnus-cache-decoded-group-names,
  and function gnus-cache-decoded-group-name.
  (gnus-cache-generate-active): Do not access
  gnus-cache-unified-group-names.
  (gnus-cache-file-name): Don't decode.