Change tzdata release to use rearguard data
Current tzdata build script uses the stock tzdata files, with local override (icuzones).
ICU TC prefers to use rearguard data. Update the build script, remove some local override rules in icuzones.
Updated the build script to generate rearguard version of zone files.
ICU tz data is still 2018i, but replaced with the version built by the new script.