RFR: 8343484: Remove unnecessary @SuppressWarnings annotations (nio)

[Archie Cobbs]

Please review this patch which removes unnecessary `@SuppressWarnings` annotations.

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Commit messages:
 - Remove unnecessary @SuppressWarnings annotations.

Changes: https://git.openjdk.org/jdk/pull/21858/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21858&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343484
  Stats: 3 lines in 3 files changed: 0 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/21858.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21858/head:pull/21858

PR: https://git.openjdk.org/jdk/pull/21858