Remove compat code and compat functions from mm-util.el

Remove compat code and compat functions from mm-util.el

* lisp/gnus/mm-util.el (mm-mime-mule-charset-alist): Remove
compat code.
(mm-coding-system-priorities)
(mm-mule-charset-to-mime-charset, mm-charset-after)
(mm-mime-charset, mm-iso-8859-x-to-15-region): Remove compat code.
(mm-detect-coding-region): Define unconditionally.
(mm-detect-mime-charset-region): Ditto.
(mm-coding-system-to-mime-charset): It's 'mime-charset now.
(coding-system-name)
(find-file-coding-system-for-read-from-filename)
(find-operation-coding-system): Remove aliases.