RFR: 8355065: ConcurrentModificationException in RichDiagnosticFormatter

[Liam Miller-Cushon]

Hi, please consider this fix for [JDK-8355065](https://bugs.openjdk.org/browse/JDK-8355065). RichDiagnosticFormatter is comparing type variables by their `toString` representation, and in this example the comparison fails due to type annotations. Replacing a call to `stripMetadataIfNeeded` with `stripMetadata` allows the comparison to succeed, and is consistent with the approach used in other parts of RichDiagnosticFormatter (e.g. for [the fix for JDK-8144580](https://github.com/openjdk/jdk/commit/570883e4dc5f769e17b3fcd8f2bbeb369dfe3a5d)).

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Commit messages:
 - 8355065: ConcurrentModificationException in RichDiagnosticFormatter

Changes: https://git.openjdk.org/jdk/pull/24769/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24769&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8355065
  Stats: 134 lines in 2 files changed: 133 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/24769.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24769/head:pull/24769

PR: https://git.openjdk.org/jdk/pull/24769