RFR: 8344336: Remove class System.CallersHolder, leftover from JEP468

Fri Nov 15 21:41:19 UTC 2024

Please review this trivial cleanup PR which removes the nested static private class `System.CallersHolder`.

This class was used pre JEP486 to track callers of `System::setSecurityManager` such that warnings were printed only once per caller.

This seems like a leftover from JEP-486 which would be nice to clean up.

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Commit messages:
 - Remove nested static private class System.CallersHolder

Changes: https://git.openjdk.org/jdk/pull/22167/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22167&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344336
  Stats: 7 lines in 1 file changed: 0 ins; 7 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/22167.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22167/head:pull/22167

PR: https://git.openjdk.org/jdk/pull/22167