docs: remove references to removed APIs

docs: remove references to removed APIs

The glib adoption docs was suggesting avoidance of certain APIs that
were obsoleted by glib, during the transition period. Now that the
referenced APIs no longer exist in libvirt code, they can also be
removed from the docs.

NB, the virStringListRemoveDuplicates method remains since there is
no glib equivalent.

Reviewed-by: Pavel Hrdina <phrdina@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>