Remove the feature flag and dead code for the old time zone picker

Created b/77277084 to help Android variants to migrate from
old time zone data source, i.e. ZoneGetter.getZonesList, to
new time zone source, e.g. TimeZoneFinder.

Bug: 72376227
Test: m RunSettingsRoboTests ROBOTEST_FILTER=com.android.settings.datetime
Test: manual
Change-Id: I332077a67cc9f9c83b298e25feea71463e1ee98b