RFR: 8348301: Remove unused Reference.waitForReferenceProcessing break-ins in tests

[Aleksey Shipilev]

`Reference.waitForReferenceProcessing` is in internal API, which should go away with [JDK-8344332](https://bugs.openjdk.org/browse/JDK-8344332). There is single use of that internal API in the tests, the last use of which was removed by [JDK-8080225](https://bugs.openjdk.org/browse/JDK-8080225). We can remove the `Reference.waitForReferenceProcessing` break-ins in the tests now.

Additional testing:
 - [x] Affected test still passes

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/23239/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23239&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8348301
  Stats: 26 lines in 1 file changed: 0 ins; 26 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/23239.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23239/head:pull/23239

PR: https://git.openjdk.org/jdk/pull/23239