RFR: 8355240: Remove unused Import in StringUTF16

[Shaojin Wen]

This PR is to remove unused imports after PR #16425

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

Commit messages:
 - remove unused import

Changes: https://git.openjdk.org/jdk/pull/24788/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24788&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8355240
  Stats: 2 lines in 1 file changed: 0 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/24788.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24788/head:pull/24788

PR: https://git.openjdk.org/jdk/pull/24788