<i18n dev> RFR: 8336300: DateFormatSymbols#getInstanceRef returns non-cached instance

[Naoto Sato]

Removing a redundant private method, which has the same implementation with the public sibling and obsolete method description.

-------------

Commit messages:
 - initial commit

Changes: https://git.openjdk.org/jdk/pull/20199/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20199&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336300
  Stats: 16 lines in 2 files changed: 0 ins; 14 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/20199.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20199/head:pull/20199

PR: https://git.openjdk.org/jdk/pull/20199