RFR: 8375350: Remove usage of AppContext from javax.imageio implementation

[Phil Race]

This PR removes usage of AppContext from classes under javax/swing/text.
Most of the uses were added in specific bug fixes 10-15 years ago but are now obsolete and the tests for those bugs need to be deleted too.

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Commit messages:
 - 8375350

Changes: https://git.openjdk.org/jdk/pull/29259/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29259&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8375350
  Stats: 387 lines in 9 files changed: 8 ins; 351 del; 28 mod
  Patch: https://git.openjdk.org/jdk/pull/29259.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29259/head:pull/29259

PR: https://git.openjdk.org/jdk/pull/29259