Don't look up DOCTYPE when resolving SDK settings on Mac OS

We don't want to hit the network, as a flakey network or slow server will
hang the XML parsing. We assume the XML is well formed.

Change-Id: Idc4898a925a46222954bf633a04ea9fe148c6797
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>