Clean up GenerateUnihanCollators.java

[markusicu]

Moved here from CLDR-9775 where @macchiati wrote in 2016:

The code is pretty crufty, since it was mostly designed to synthesize data from different sources before kMandarin and kTotalstrokes were expanded in Unihan.

The code can be simplified, and can also be expanded to generate whole files, instead of having people cut and paste.

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In 2019, Mark passed it on to @pedberg-icu who today assigned it to me...

[pedberg-icu]

Also see the CLDR dup: https://unicode-org.atlassian.net/browse/CLDR-9932