RFR: 8304912: Use OperatingSystem enum in java.desktop module [v8]

[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.

Roger Riggs has updated the pull request incrementally with one additional commit since the last revision:

  Remove unused getSystemProperty method and unused imports

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13213/files
  - new: https://git.openjdk.org/jdk/pull/13213/files/a3334e88..b2aca952

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13213&range=07
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13213&range=06-07

  Stats: 13 lines in 3 files changed: 0 ins; 13 del; 0 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