RFR: 8378193: Remove AppContext from JinternalFrame

[Phil Race]

We don't need a per-AppContext JInternalFrame focusListener any more.

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Commit messages:
 - 8378193

Changes: https://git.openjdk.org/jdk/pull/29799/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29799&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8378193
  Stats: 11 lines in 1 file changed: 0 ins; 5 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/29799.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29799/head:pull/29799

PR: https://git.openjdk.org/jdk/pull/29799