Use PropertyValueAliases for the script mapping.

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.
Remove the IANA subtag registry parser

This is not needed now that PropertyValueAliases is used for script
mapping.