Remove unused fields in ThaiBuddhistChronology

There are 3 static HashMap's in ThaiBuddhistChronology class:
ERA_NARROW_NAMES, ERA_SHORT_NAMES, ERA_FULL_NAMES.
static initializer put some values into them. But their content unused after that.
Initially reported here https://mail.openjdk.java.net/pipermail/core-libs-dev/2021-September/081866.html