Remove XEmacs compat functions from mm-util.el

Remove XEmacs compat functions from mm-util.el

* lisp/gnus/mm-util.el (mm-special-display-p): Remove.
(mm-decode-coding-string, mm-encode-coding-string)
(mm-decode-coding-region, mm-encode-coding-region): Remove.
(mm-string-to-multibyte): Remove.
(mm-char-or-char-int-p): Remove.
(mm-ucs-to-char): Remove compat versions of the function.
(mm-read-coding-system): Remove.
(mm-coding-system-p): Remove compat code.
(mm-enrich-utf-8-by-mule-ucs): Remove.
(mm-enable-multibyte, mm-disable-multibyte): Remove compat versions.
(mm-delete-duplicates): Remove.
(mm-multibyte-p): Remove compat versions.
(mm-xemacs-find-mime-charset-1): Remove.
(mm-xemacs-find-mime-charset): Remove.
(mm-make-temp-file): Made obsolete.
(mm-find-buffer-file-coding-system): Remove XEmacs compat.