RFR: 8376996: Remove AppContext usage from SunClipboard.java

[Phil Race]

Remove AppContext from SunClipboard.

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Commit messages:
 - 8376996

Changes: https://git.openjdk.org/jdk/pull/29533/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29533&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8376996
  Stats: 112 lines in 3 files changed: 13 ins; 72 del; 27 mod
  Patch: https://git.openjdk.org/jdk/pull/29533.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29533/head:pull/29533

PR: https://git.openjdk.org/jdk/pull/29533