docs/tools/ttl2sgml: Don't error if we have nao:deprecated for namespaces

Previously, this would error when generating the documentation with something
like:

(lt-ttl2sgml:4857): Tracker-ERROR **:
'http://www.tracker-project.org/temp/mlo#' is not a class nor a property!?

and

../../../docs/tools/gen-doc.sh: line 52:  4857 Trace/breakpoint trap   (core
dumped) $TTL2SGML -d $f -o $BUILD_DIR/$PREFIX-ontology.xml -f
$BUILD_DIR/fts-properties.xml -e $ONTOLOGIES_INFO_DIR/$PREFIX/explanation.xml

Given the entire namespace for MLO is deprecated, we have simply removed the
g_error here.