Remove no-op calls to decode-char with 'ucs' arg

Remove no-op calls to decode-char with 'ucs' arg

* lisp/gnus/mm-util.el (mm-ucs-to-char):
* lisp/language/hanja-util.el (hangul-to-hanja-char):
* lisp/leim/quail/hangul.el (hangul3-input-method-internal)
(hangul390-input-method-internal):
* lisp/nxml/rng-cmpct.el (rng-c-process-escapes):
* lisp/nxml/xsd-regexp.el (xsdre-compile-single-char)
(xsdre-range-list-to-char-alternative):
* lisp/xml.el (xml-parse-string, xml--entity-replacement-text)
(xml-substitute-special): Remove calls to decode-char where first
argument is 'ucs'; that is now a no-op.  Discussed in Bug#52263.

* lisp/nxml/xmltok.el (xmltok-unicode-to-char): Make into obsolete
function alias for 'identity'.  Update single caller.