Remove Gnus XEmacs compatibility

Remove Gnus XEmacs compatibility

* lisp/gnus/gnus-demon.el (gnus-demon-idle-since): Remove
XEmacs compat.

* lisp/gnus/gnus-dired.el: Remove XEmacs compat.

* lisp/gnus/gnus-draft.el: Remove XEmacs compat.

* lisp/gnus/gnus-gravatar.el (gnus-gravatar-insert): Remove
XEmacs compat.

* lisp/gnus/gnus-group.el: Remove XEmacs compat.

* lisp/gnus/gnus-html.el: Remove XEmacs compat.

* lisp/gnus/gnus-ml.el: Remove XEmacs compat.

* lisp/gnus/gnus-picon.el (gnus-picon-style): Remove XEmacs comment.