Simplify locale setup

Simplify locale setup

Stop exporting Vprevious_system_time_locale and
Vprevious_system_messages_locale to Elisp.  I did that export by
mistake in 1999, and the Elisp variables have never been used.
Simplifying this cruft should make it easier to fix Bug#39248.
* etc/NEWS: Mention this.
* src/emacs.c (main): Simplify locale initialization.
(synchronize_locale): Simplify.
(Vprevious_system_time_locale, Vprevious_system_messages_locale):
Now static variables not visible to Lisp, and defined only if
HAVE_SETLOCALE.
(Vprevious_system_messages_locale): Define only if LC_MESSAGES.