RFR: 8275197: Remove unused fields in ThaiBuddhistChronology

[Andrey Turbanov]

Remove 3 unused HashMap's.
Reported here https://mail.openjdk.java.net/pipermail/core-libs-dev/2021-September/081866.html
I did the similar PR to treetenbp and it was merged https://github.com/ThreeTen/threetenbp/pull/155

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Commit messages:
 - [PATCH] Remove unused fields in ThaiBuddhistChronology

Changes: https://git.openjdk.java.net/jdk/pull/5917/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5917&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275197
  Stats: 37 lines in 1 file changed: 0 ins; 36 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5917.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5917/head:pull/5917

PR: https://git.openjdk.java.net/jdk/pull/5917