Remove compat code for older Emacsen

Remove compat code for older Emacsen

* lisp/gnus/gnus-art.el (gnus-button-url-regexp): Remove
XEmacs compat code.

* lisp/gnus/gnus-sync.el (gnus-sync-json-alist-p): Remove
unused compat function.
(gnus-sync-json-plist-p): Ditto.

* lisp/gnus/message.el (message-default-charset): Make obsolete.
(message-info): Remove compat code.
(message-setup-fill-variables): Remove kludge needed earlier
to not overwrite `normal-auto-fill-function'.
(message-split-line): Remove compat code.

* lisp/gnus/mm-view.el (mm-display-inline-fontify): Remove
compat code.