virXMLFormatMetadata: Don't modify deprecated 'xmlIndentTreeOutput'

virXMLFormatMetadata: Don't modify deprecated 'xmlIndentTreeOutput'

'libxml2' deprecated the 'xmlIndentTreeOutput' thread-local variable as
well as the 'xmlThrDefIndentTreeOutput' function for setting the global
default, which we use in our code for formatting the metadata sub-XML.

'libxml2' also for now doesn't provide a way to set target indentation
level in 'xmlSaveCtxt' which would allow us to use the modern output
APIs, we can't replace our use of 'xmlDumpNode'. (See
https://gitlab.gnome.org/GNOME/libxml2/-/issues/989 )

Since the indentation is enabled by default in libxml2 and our most
commonly used code which calls xmlDumpNode lives in a standalone
process, where we don't override the setting, just removing the override
will result in identical behaviour.

For the use cases which do live in a process we don't fully control and
thus the default could have been overriden, the result would be that the
<metadata> element would be un-indented, but that is still valid XML.

Thus to fix the deprecated use just stop setting 'xmlIndentTreeOutput'.

Closes: https://gitlab.com/libvirt/libvirt/-/issues/816
Signed-off-by: Peter Krempa <pkrempa@redhat.com>