RFR: 8376232: Remove AppContext from Swing synth related classes

[Phil Race]

Remove AppContext from Swing's synth related classes.
2 tests that check this code uses AppContext are removed.

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Commit messages:
 - 8376232

Changes: https://git.openjdk.org/jdk/pull/29394/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29394&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8376232
  Stats: 313 lines in 8 files changed: 6 ins; 283 del; 24 mod
  Patch: https://git.openjdk.org/jdk/pull/29394.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29394/head:pull/29394

PR: https://git.openjdk.org/jdk/pull/29394