Get rid of StorageLocations concept

The StorageLocations concept was introduced to support a feature
of the qtjsondb backends, which were never released and have now
been removed. The concept is no longer desired, as the semantics
are unclear and no existing backend provides the feature.

Change-Id: I4b1826c8e9562bc4e6c8d93935909b8b8089f29f
Reviewed-by: Christopher Adams <chris.adams@jollamobile.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>
Reviewed-by: Michael Krasnyk <michael.krasnyk@gmail.com>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>