<i18n dev> RFR: 8312521: Unused field LocaleProviderAdapter#defaultLocaleProviderAdapter could be removed

[Andrey Turbanov]

After [JDK-8245241](https://bugs.openjdk.org/browse/JDK-8245241), the field `sun.util.locale.provider.LocaleProviderAdapter#defaultLocaleProviderAdapter` is only written in `<clinit>` and then is not used after.
We can remove it.

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Commit messages:
 - 8312521: Unused field LocaleProviderAdapter#defaultLocaleProviderAdapter could be removed

Changes: https://git.openjdk.org/jdk/pull/15388/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15388&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8312521
  Stats: 11 lines in 1 file changed: 0 ins; 8 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/15388.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15388/head:pull/15388

PR: https://git.openjdk.org/jdk/pull/15388