RFR: 8376420: Remove AppContext from javax/swing/ImageIcon.java

[Phil Race]

Remove per-AppContext MediaTracker from ImageIcon.java

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Commit messages:
 - 8376420

Changes: https://git.openjdk.org/jdk/pull/29433/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29433&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8376420
  Stats: 20 lines in 1 file changed: 1 ins; 16 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/29433.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29433/head:pull/29433

PR: https://git.openjdk.org/jdk/pull/29433