RFR: 8304912: Use OperatingSystem enum in java.desktop module

[Roger Riggs]

Update classes in the java.desktop module to use the jdk.internal.util.OperatingSystem enum instead of the `os.name` system property to select OS specific behaviors.

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Commit messages:
 - Remove unused os.arch system property
 - Apply OperatingSystem enum instead of os.name to java.desktop module

Changes: https://git.openjdk.org/jdk/pull/13213/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13213&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304912
  Stats: 105 lines in 15 files changed: 19 ins; 39 del; 47 mod
  Patch: https://git.openjdk.org/jdk/pull/13213.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13213/head:pull/13213

PR: https://git.openjdk.org/jdk/pull/13213