RFR: 8308843: Generational ZGC: Remove gc/z/TestHighUsage.java

[Axel Boldt-Christmas]

`gc/z/TestHighUsage.java` was originally used to test the high usage heuristics rule for single generational ZGC. For generational ZGC this test does not test anything concrete and mostly produces noise. So it should be removed.

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Commit messages:
 - 8308843: Generational ZGC: Remove gc/z/TestHighUsage.java

Changes: https://git.openjdk.org/jdk/pull/15192/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15192&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308843
  Stats: 101 lines in 1 file changed: 0 ins; 101 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15192.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15192/head:pull/15192

PR: https://git.openjdk.org/jdk/pull/15192