libxml2 2.12.0 made `xmlGetLastError()` return `const` pointer:

https://gitlab.gnome.org/GNOME/libxml2/-/commit/61034116d0a3c8b295c6137956adc3ae55720711

Clang 16 does not like this:

    error: assigning to 'xmlErrorPtr' (aka '_xmlError *') from 'const xmlError *' (aka 'const _xmlError *') discards qualifiers
    error: cannot initialize a variable of type 'xmlErrorPtr' (aka '_xmlError *') with an rvalue of type 'const xmlError *' (aka 'const _xmlError *')

Let’s update the variables to `const`.
For older versions, it will be automatically converted.

But then `xmlResetError(xmlError*)` will not like the `const` pointer:

    error: no matching function for call to 'xmlResetError'
    note: candidate function not viable: 1st argument ('const xmlError *' (aka 'const _xmlError *')) would lose const qualifier

Let’s replace it with `xmlResetLastError()`.

ALso remove `LIBXMLDOC::Xerr` protected member property.
It was introduced in 65b0e545
along with the `xmlResetError` calls.
It does not appear to be used for anything.