RFR: 8378605: Remove AppContext from the Swing TimerQueue implementation

[Phil Race]

Remove per-AppContext TimerQueue support. Just create one for the VM.
A test that expressly checked per-AppContext TimerQueue behaviour is removed.

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Commit messages:
 - 8378605

Changes: https://git.openjdk.org/jdk/pull/29902/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29902&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8378605
  Stats: 193 lines in 2 files changed: 1 ins; 185 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/29902.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29902/head:pull/29902

PR: https://git.openjdk.org/jdk/pull/29902