MDL-76537 core: Fix date_format_string() to work with php-intl
With Moodle 4.1 and up, we have stopped to work with strftime(),
because it's deprecated for PHP 8.1 and up and, instead we are
using core_date::strftime() that performs the conversions using
the php-intl extension.
The original strftime() had a lot of OS-specific dependencies,
both about locale names and charsets used to provide the information.
Instead, the php-intl gets rid of all those OS-specific nightmares
and only uses ICU/Unicode locales and UTF-8.
Because of that, the old utf-8 => windows charset => utf-8
conversions aren't needed anymore. Hence, removing them.