RFR: 8303024: Use Set.of in WindowsFileSystem

[Glavo]

Simply clean up. I need someone to help me open an issue on JBS, thanks.

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Commit messages:
 - Use Set.of in WindowsFileSystem

Changes: https://git.openjdk.org/jdk/pull/12697/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12697&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303024
  Stats: 3 lines in 1 file changed: 0 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/12697.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12697/head:pull/12697

PR: https://git.openjdk.org/jdk/pull/12697