RFR: 8326941: Remove StringUTF16::isBigEndian

[Per Minborg]

This PR proposes to remove the native method `StringUTF16::isBigEndian` and its corresponding C implementation and instead rely on the `jdk.internal.misc.Unsafe::isBigEndian` method.

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Commit messages:
 - Update copyright years
 - Remove native method

Changes: https://git.openjdk.org/jdk/pull/18348/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18348&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8326941
  Stats: 18 lines in 2 files changed: 2 ins; 13 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/18348.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18348/head:pull/18348

PR: https://git.openjdk.org/jdk/pull/18348