(Bug 48117) In 1.20, don't try to set defaultContentModel

MWNamespace::getNamespaceContentModel() is not present in MediaWiki 1.20, but
neither is anything else that would care for 'defaultContentModel'. Removing
the call prevents a crash.

Change-Id: Ib7f793c8559233aa1e7a128a03d133795cb06402