MeasureNumbering might be dead code

[webern]

See if anything is actually implementing MeasureNumbering. If not, remove it.

The enum is in ApiCommon.h

        enum class MeasureNumbering
        {
            unspecified,
            none,
            measure,
            system
        };

It is used in MeasureData and DefaultsData but unclear if anything reads or writes it to/from MusicXML.