automake: remove unused Automake::FileUtils::up_to_date_p function.

automake: remove unused Automake::FileUtils::up_to_date_p function.

Per thread at:
https://lists.gnu.org/archive/html/automake-patches/2020-04/msg00000.html>
especially:
https://lists.gnu.org/archive/html/automake-patches/2020-05/msg00003.html

* lib/Automake/FileUtils.pm (up_to_date_p): remove.
Nothing in Automake itself uses this.  It is used in Autoconf's
autom4te utility, but Autoconf has its own copy, and the duplication
was impeding auto4mte development, as discussed in the thread above.
(While here, insert missing =over/=back to placate pod2text.)
* NEWS (Miscellanous changes): note this.