The mapping of the script labels in the UCD data to ISO 15924
script tags is now done using the sc property map in the
PropertyValueAliases data.

This has the following benefits:

    1.  It removes the dependency on the IANA subtag registry.

    2.  It ensures the scripts are correct as specified in the
        UCD data files.

This removes support for the CSUR (ConScript Unicode Registry) data
in the main Unicode APIs. This data should be accessed through a
different API.