use `xmlFreeNode` in xml_document.c to deallocate unparented nodes of all types

[flavorjones]

As @stevecheckoway pointed out in #1784 we might be able to avoid the case statement in dealloc_node_i and let xmlFreeNode take care of it for us.

This change would be a simplification, but I need to spend some time digging into:

• why we call xmlFreePropList on XML_ATTRIBUTE_NODEs
• why we reparent most nodes back into the document instead of just freeing them.

Both of these features have been around since 2009 (1aa012a) and I couldn't quickly find a clear reason why. Let's be careful here not to introduce an inadvertent memory leak.

[flavorjones]

@rubarb666 can you explain a bit more about your comment?

[flavorjones]

I'd like to block this on #1603 which will add the memory leak test suite to CI

[flavorjones]

Hey! The memory leak suite has been back in effect since 6326caa (2023-12-10) so let's do this.

[flavorjones]

Yeah, so this didn't work -- it either leaked memory or segfaulted. I'm not going to spend time on figuring out why, though, since the code it would simplify is pretty minimal and hopefully more generally we can get rid of the nonstandard memory lifecycle of nodes (as Nick suggested in #1952 (comment)).