bug#52263: Stale comment in xsd-regexp.el about Emacs not supporting Unicode

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Kangas <stefan@marxist.se>
> Date: Fri, 3 Dec 2021 10:37:10 -0800
> 
> I believe this comment in lisp/nxml/xsd-regexp.el can be removed as
> Emacs supports Unicode now:
> 
>     ;; The semantics of XSD regexps are defined in terms of Unicode.
>     ;; Non-Unicode characters are not allowed in regular expressions and
>     ;; will not match against the generated regular expressions.  A
>     ;; Unicode character means a character in one of the Mule charsets
>     ;; ascii, latin-iso8859-1, mule-unicode-0100-24ff,
>     ;; mule-unicode-2500-33ff, mule-unicode-e000-ffff, eight-bit-control
>     ;; or a character translatable to such a character (i.e a character
>     ;; for which `encode-char' will return non-nil).
>     ;;
>     ;; Unfortunately, this means that this package is currently useless
>     ;; for CJK characters, since there's no mule-unicode charset for the
>     ;; CJK ranges of Unicode.  We should devise a workaround for this
>     ;; until the fabled Unicode version of Emacs makes an appearance.
> 
> Is that correct?

Probably.  The mule-Unicode-* stuff is definitely obsolete.  The only
thing that bothers me is what happens with eight-bit characters in the
XSD regexps -- are they allowed?  Emacs in general does allow them.
If xsd-regexp.el doesn't, that should be stated there.