Remove checks for locale.h, setlocale, localeconv

Remove checks for locale.h, setlocale, localeconv

The `<loccale.h>` header file, setlocale, and localeconv are part of the
standard C89 [1] and on current systems can be used unconditionally.

Since PHP 7.4 requires at least C89 or greater, the `HAVE_LOCALE_H`,
`HAVE_SETLOCALE`, and `HAVE_LOCALECONV` symbols defined by Autoconf in
configure.ac [2] can be ommitted and simplifed.

The bundled libmagic (file) has also been patched already in version
5.35 and up in upstream location so when it will be patched also in
php-src the check for locale.h header is still left in the configure.ac
and in windows headers definition file.

[1] https://port70.net/~nsz/c/c89/c89-draft.html#4.4
[2] https://git.savannah.gnu.org/cgit/autoconf.git/tree/lib/autoconf/headers.m4

Omit the bundled libmagic files